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gistfile1.c

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

int main(int argc, char **argv) {
    long int numbers[5];
    long int highest = 0;
    if (argc < 2 || strlen(argv[1]) < 5) {
        fprintf(stderr, "5 digits minimum input string must be given\n");
        return EXIT_FAILURE;
    }
    for (char *c = argv[1], *s = c; *c; ++c) {
        if (!isdigit(*c)) {
            fprintf(stderr, "Input string contains characters\n");
            return EXIT_FAILURE;
        }
        numbers[c-s] = *c - '0';
        if (c-s == 4) {
            long int result = numbers[0];
            for (int i = 1; i < 5; ++i) result *= numbers[i];
            if (result > highest) highest = result;
            c = s++;
        }
    }
    printf("%ld\n", highest);
    return EXIT_SUCCESS;
}