样例没问题，但是全WA，求助大佬

#define _CRT_SECURE_NO_WARNINGS 1
#include<iostream>
#include<string.h>
using namespace std;
int main()
{
    char y[27] = "abcdefghijklmnopqrstuvwxyz";
    int n;
    cin >> n;
    getchar();//吞掉回车
    char a[55];
    cin.getline(a, 55);
    int len = strlen(a);
    for (int i = 0;i < len;i++) {
        for (int k = 0;k < 26;k++) {
            if (a[i] == y[k]) {
                a[i] = y[(k+=n) % 26];
            }
        }
    }
    cout << a;
    return 0;
}