Use sentinel value

Problem becomes: 

Use sentinel value

#include <stdio.h>

int main()
{
float average; /* new data type */
int counter, grade, total;

/* initialization phase */
total = 0;
counter = 0;

/* processing phase */
printf( "Enter grade, -1 to end: " );
scanf( "%d", &grade );

while ( grade != -1 ) {
total = total + grade;
counter = counter + 1;
printf( "Enter grade, -1 to end: " );
scanf("%d", &grade);
}

/* termination phase */
if ( counter != 0 ) {
average = ( float ) total / counter;
printf( "Class average is %.2f", average );
}
else
printf( "No grades were entered\n" );

return 0; /* indicate program ended successfully */
}