3，4不通过，哪位大佬能帮忙看看有什么问题吗？

#include<stdio.h>
#include<string.h>
#include<math.h>

char a[105];
int main()
{
    int n;
    scanf("%d", &n);
    scanf("%s", a);
    for (unsigned int i = 0; i < strlen(a); i++)
    {
        a[i] = a[i] + n;
        while (a[i] > 'z')
            a[i] = a[i] - 26;
        printf("%c", a[i]);
    }
    return 0;
}