新手求助大佬

# include <iostream>
# include <algorithm>
# include <string>
# include <functional>
# include <vector>
# include <cstring>
# include <cmath>
# include <cstdio>
# include <queue>
# include <cstdlib>
using namespace std;

int main()
{
    int k;
    cin >> k;
    string s;
    cin >> s; 
    int l = s.size();
    for (int i = 0; i <= l - 1; i++)
    {
        if (s[i] + k >= 'z') cout << char(s[i] + k - 36);
        else cout << char(s[i] + k);
    }
    return 0;
}

不知道为什么40分，自己测试的时候还正常，大佬帮忙看看