求教，为什么80分过不去？

#include<cstdio>
#include<iostream>
#include<cstring>
using namespace std;
char str[1000];
int n, i, x, s;
int main()
{
    cin >> n;
    scanf("%s", &str);
    if (n == 26) {
        cout << str;
        return 0;
    }
    x = strlen(str);
    for (i = 0;i < x;i++) {
        if (str[i] >= 'A' && str[i] <= 'Z') str[i] = char(int(str[i]) + 'a' - 'A');
        if (str[i] == 'z') str[i] = char(96);
        s = int(str[i]) + n % 26;
        if (s > 122) s = s - 122 + 96;
        str[i] = char(s);
    }
    cout << str;
    return 0;
}