为什么80？

#include<cstdio>
using namespace std;
int main()
{
    int n;
    char a[999999];
    scanf ("%d%s",&n,a);
    for (int i=0;a[i]!='\0';i++)
    {
        a[i]+=n%26;
        putchar (a[i]);
    }
    return 0;
}