this program gets WA#10.
if you consider this program wrong, say whats wrong.

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

int n;
char s1[500100];
char s2[250100];
const int p1 = 19;
const int p2 = 683;
int h[250100];

int strstr()
{
    // int n = strlen(s1);
    int i, t = 0, p = 1;
    for (i=0;i<n;i++)
    {
        t = (t*p1 + s1[i]-33)%p2;
        p = (p)*p1%p2;
    }
    h[0] = t;
    for (i=1;i<=n;i++)
    {
        int v = ((h[i-1]*p1-(s1[i-1]-33)*p)+s1[i+n-1]-33)%p2;
        if (v<0) v = v+p2;
        h[i] = v;
    }
//    for (i=0;i<n;i++) printf("%d\n",h[i]);
    t = 0;
    for (i=0;i<n;i++) t = (t*p1 + s2[i]-33)%p2;
    for (i=0;i<=n;i++)
        if (t==h[i])
            if (strncmp(s1+i,s2,n)==0)    return i;
    return -1;
}

int main()
{
    scanf("%d",&n);
    scanf("%s",s1);
    scanf("%s",s2);
    strncpy(s1+n,s1,n);
    s1[2*n] = 0;
    int t = strstr();
//    printf("%d\n",t-reinterpret_cast<int>(s1));
    if (t==-1) printf("-1\n");else printf("%d\n",(n-t)%n);
    return 0;
}