为啥它总有一个会RE？

P1914 小书童——密码 求救！

#include <iostream>
#include <cstring>
using namespace std;
int main()
{
    string article,password;
    int n,i,j;
    cin>>n;
    cin>>article;
    for(i=0;i<article.length();i++)
    {
        if(article[i]+n<='z') password[i]=article[i]+n;
        else password[i]='a'-1+n-('z'-article[i]);
        }
    for(i=0;i<article.length();i++) cout<<password[i];
    return 0;
}