为什么一个测试点都过不了 ！！！

#include <iostream>
using namespace std;
const int n = 1001;
int num[n];
int main() {
    int a, ans = 0, max = 0;
    cin >> a;
    for (int i = 1; i <= a; i++) {
        cin >> num[i];
    }
    for (int i = 1; i <= a - 1; i++) {
        if (num[i + 1] - num[i] == 1)ans++;
        else {
            if (ans >= max) max = ans;
            ans = 0;
        }
    }
    cout << max;
    return 0;
}

救救我！谢谢！！