40分求助

#include<iostream>
#include<cstdio>
#include<cstring>
#include<string>
#include<algorithm>
using namespace std;
char mode[1000]; 
int main()
{int n,i,j;
cin>>n;
scanf("%s",mode);
i=strlen(mode);
for(j=0;j<i;j++)
   if(mode[j]=='z'){mode[j]-=26;mode[j]+=n;}
   else mode[j]+=n;
printf("%s",mode);
return 0;
}