3_2.c

// Инструкция повторения, управляемая сигнальным значением
#include <stdio.h>

int main(void)
{
    unsigned int counter;
    int grade;
    int total;

    float average;

    total = 0;
    counter = 0;

    printf("%s", "Enter grade, -1 to end: ");
    scanf("%d", &grade);

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

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