indianCurrencyFormatter.c

#include <stdio.h>
#include <string.h>

int main(void) {
    int len,i;
    char str[100]; 
    gets(str);
    len = strlen(str);
    if (len < 3) {
        printf("%s",str);
    }
    if (len%2 == 0) {
        printf("%c,",str[0]);
        for(i=1; i<len-3; i+=2){
            printf("%c%c,",str[i],str[i+1]);
        }
    } else {
        for(i=0; i<len-4; i+=2){
            printf("%c%c,",str[i],str[i+1]);
        }
    }
    printf("%c%c%c",str[len-3],str[len-2],str[len-1]);
    return 0;
}

1. didn't handle leading zeros

Good @fakruboss