C Program to calculate grade based on percentage using switch statement

Program

#include<stdio.h>
#include<stdlib.h>
void main()
{
    long int m1, m2, m3, m4, m5, m6;
    long int percent;
    printf("Enter the marks for 6 subjects\n");
    scanf("%ld %ld %ld %ld %ld %ld", &m1, &m2, &m3, &m4, &m5, &m6);
    percent = ((m1 + m2 + m3 + m4 + m5 + m6) * 100 / 600);
    printf("Percentage : %ld\n", percent);
    switch (percent)
    {
    case 90 ... 100:
        printf("S grade\n");
        break;
    case 80 ... 89:
        printf("A grade\n");
        break;
    case 70 ... 79:
        printf("B grade\n");
        break;
    case 60 ... 69:
        printf("C grade\n");
        break;
    case 50 ... 59:
        printf("D grade\n");
        break;
    case 40 ... 49:
        printf("E grade\n");
        break;
    case 0 ... 39:
        printf("F grade\n");
        break;
    default:
        printf("Invalid input");
        exit(0);
    }
}

Output 1

$ gcc grade-based-on-percent-switch.c
$ ./a.out
Enter the marks for 6 subjects
98
95
93
99
96
97
Percentage : 96
S grade

Output 2

$ gcc grade-based-on-percent-switch.c
$ ./a.out
Enter the marks for 6 subjects
12
30
42
9 
25
32
Percentage : 25
F grade