1-72 convert string to decimal integer

my Code  

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

int main() {
    char c;
    int len = 0, flag = 0, a[8] = {0};
    long int ans = 0;
    while ((c = getchar()) != '#') {
        if (c == '-')
            flag = 1;
        if (c >= '0' && c <= '9') {
            a[len++] = c - '0';
            break;
        }
        if (c >= 'a' && c <= 'f') {
            a[len++] = c - 'a' + 10;
            break;
        }
        if (c >= 'A' && c <= 'F') {
            a[len++] = c - 'A' + 10;
            break;
        }
    }
    if (c != '#')
        while ((c = getchar()) != '#') {
            if (c >= '0' && c <= '9')
                a[len++] = c - '0';
            if (c >= 'a' && c <= 'f')
                a[len++] = c - 'a' + 10;
            if (c >= 'A' && c <= 'F')
                a[len++] = c - 'A' + 10;
        }
    for (int i = len - 1, k = 1; i >= 0; --i, k *= 16)
        ans += a[i] * k;
    if (flag)
        ans *= -1;
    printf("%ld", ans);
    return 0;
}