最后一个点wa

#include<bits/stdc++.h>
using namespace std;
const int maxn = 2010;
int a[maxn] = {0}, b[maxn] = {0}, c[maxn * 2] = {0};
int main() {
    string s1, s2;
    cin >> s1 >> s2;
    int l1 = s1.length(), l2 = s2.length();
    for (int i = 0; i < l1; i++) a[i] = s1[l1 - 1 - i] - '0';
    for (int i = 0; i < l2; i++) b[i] = s2[l2 - 1 - i] - '0';

    for (int i = 0; i < l1 + l2 - 1; i++) {
        for (int j = 0; j <= i; j++) {
            c[i] += a[j] * b[i - j];
        }

        if (c[i] >= 10) {
            c[i + 1] += c[i] / 10;
            c[i] = c[i] % 10;
        }
    }

    int mark = maxn * 2;
    for (; mark >= 1; mark--) {
        if (c[mark] != 0) break;
    }
    for (; mark >= 0; mark--) {
        cout << c[mark];
    }
    return 0;
}

实在不知道哪里错了。。。