两个测试点没过，求解答

#include <stdio.h>
#include <string.h>
int main ()
{
    char a[51];
    int n;
    scanf ("%d",&n);
    scanf ("%s",a);
    int len = strlen(a);
    for (int i=0;i<len;i++)
    {
        a[i]=a[i]+n;
        if (a[i]>'z')
        {a[i]=a[i]-26;}
    }
    printf ("%s",a);
}