/*  Simple program to add two integers together. */
#include  <stdio.h>

int main ( ) 
{
   int number1, number2;  			/* variables to hold the two integers */
   int sum;					/* variable to hold the sum */

   printf("Enter the first number: ");		/* print a prompt */
   scanf("%d", &number1);				/* read a number from the keyboard */

   printf("Enter the second number: ");
   scanf("%d", &number2);

   sum = number1 + number2;

   printf("The sum of %d and %d is %d.\n", number1, number2, sum);

   return 0;  /* 0 indicates program ended successfully */
}  /* end main */

Compiling and running the program under Linux would look like:



$ emacs ex1.c
$ gcc -o ex1 ex1.c
$ ./ex1
Enter the first number: 5
Enter the second number: 10
The sum of 5 and 10 is 15.


NOTES about the program:
a)  "#include <stdio.h>  is a directive to the C preprocessor to include the standard input/output header file.
b)  A functions in C returns a value.  "main" is a special function where C programs start execution.  The value returned by the main function indicates the status of execution.  
c)   When a function starts execution, a chuck of memory (called a "call frame") is allocated to hold its local variable (e.g., number1, number2, sum are named locations in memory).
d)  C programming instructions/statements are terminated by a semicolon (";").  
e)  The "printf" module is sent ("passed") parameters/arguments to be printed.  Double quotes("") indicate a character string to be printed with formatting information (%d) where variables values should go.  The actual variables corresponding to the formatting information are listed after the character string.  The "\n" is the escape sequence for the non-printable "newline" character.
f)  The "scanf" module reads a specified type of data (%d for integer).  Additionally, scanf needs the memory address of the variable where the datum is to be placed -- hence the "&" address-of operator.

/*  Simple program to print a grade bases on 10 points off the top score. */
#include  <stdio.h>

int main ( )
{
  int highScore, minA, minB, minC, minD;
  int scoreToAssign;

  printf("Enter the highest score: ");    /* print a prompt */
  scanf("%d", &highScore);                /* read highest score */

  /* Calculate grade cutoffs */
  minA = highScore - 10 + 1;
  minB = minA - 10;
  minC = minB - 10;
  minD = minC - 10;

  printf("Enter the score to be assigned: ");    /* print a prompt */
  scanf("%d", &scoreToAssign);                   /* read score to be assigned*/

  if (scoreToAssign >= minA) {
    printf("The score of %d receives an A.\n", scoreToAssign);
  } /* end if */
  if (scoreToAssign >= minB && scoreToAssign < minA) {
    printf("The score of %d receives a B.\n", scoreToAssign);
  } /* end if */
  if (scoreToAssign >= minC && scoreToAssign < minB) {
    printf("The score of %d receives the grade of C.\n", scoreToAssign);
  } /* end if */
  if (scoreToAssign >= minD && scoreToAssign < minC) {
    printf("The score of %d receives the grade of D.\n", scoreToAssign);
  } /* end if */
  if (scoreToAssign < minD) {
    printf("The score of %d receives the grade of F.\n", scoreToAssign);
  } /* end if */

  return 0;  /* 0 indicates program ended successfully */
} /* end main */

/*  Simple program to print a grade bases on 10 points off the top score. */
#include  <stdio.h>

int main ( )
{
  int highScore, minA, minB, minC, minD;
  int scoreToAssign;

  printf("Enter the highest score: ");    /* print a prompt */
  scanf("%d", &highScore);                /* read highest score */

  /* Calculate grade cutoffs */
  minA = highScore - 10 + 1;
  minB = minA - 10;
  minC = minB - 10;
  minD = minC - 10;

  printf("Enter the score to be assigned: ");    /* print a prompt */
  scanf("%d", &scoreToAssign);                   /* read score to be assigned*/

  if (scoreToAssign >= minA) {
    printf("The score of %d receives an A.\n", scoreToAssign);
  } else {
    if (scoreToAssign >= minB) {
      printf("The score of %d receives a B.\n", scoreToAssign);
    } else {
      if (scoreToAssign >= minC) {
        printf("The score of %d receives the grade of C.\n", scoreToAssign);
      } else {
        if (scoreToAssign >= minD) {
          printf("The score of %d receives the grade of D.\n", scoreToAssign);
        } else {
          printf("The score of %d receives the grade of F.\n", scoreToAssign);
        } /* end if minD */
      }/* end if minC */
    }/* end if minB */
  }/* end if minA */

  return 0;  /* 0 indicates program ended successfully */
} /* end main */