40分求助

#include<bits/stdc++.h>
#include<cstring> 
using namespace std;
char str[51];
int main(){
    int a ;
    cin >> a;
    cin >> str;
    for (int i = 0;;i ++){
        if (str[i] != '\0'){
            if (str[i] == 'z'){
                cout << "a";
            }
            else{
                 str[i] += a; 
                 cout << char(str[i]);  
            }
}
        else {
            break;
}
}
    return 0;
}