试试这个

#include <iostream>
using namespace std;
int main(){
	int n;
    string s;
    cin>>n>>s;
    for (int i=0;i<s.size();i++){
        if (s[i]+n>'z') s[i]=s[i]-26+n;
        else s[i]+=n;
    }
    cout<<s;
    return 0;
}