//#define LOCAL
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAXN 250000+10

int N;
char str1[MAXN], str2[MAXN];
char temp[MAXN*2];

int kmp(char* s, char* t, int pos);
void get_next(char* t, int* next, int t_len);
int main()
{
#ifdef LOCAL
    freopen("1423_String_Tale_2.in", "r", stdin);
    freopen("1423_String_Tale_2.out", "w", stdout);
#endif

    scanf("%d", &N);
    scanf("%s%s", str1, str2);

    int s1_len = strlen(str1);
    int s2_len = strlen(str2);

    if(s1_len != s2_len){
        printf("%d\n", -1);
        return 0;
    }

    strcpy(temp, str1);
    strcat(temp, str1);
    int ans;
    ans = kmp(temp, str2, 0);
    if(ans != 0 && ans != -1)
        printf("%d\n", s1_len-ans);
    else
        printf("%d\n", ans);

    return 0;
}

int kmp(char* s, char* t, int pos)
{
    int s_len = strlen(s);
    if(pos<0 || pos>s_len-1){
        fprintf(stderr, "%s", "Error: invalid pos.");
        system("pause");
    }

    int t_len = strlen(t);
    int *next = (int*)malloc(sizeof(int) * t_len);
    get_next(t, next, t_len);

    int i = pos;
    int j = 0;

    while(i<s_len && j<t_len){
        if(j<0 || s[i]==t[j]){
            i++;
            j++;
        }
        else
            j = next[j];
    }

//    free(next);
    if(j >= t_len)
        return i-t_len;
    else
        return -1;
}

void get_next(char* t, int* next, int t_len)
{
    next[0] = -1;
    int j = 0;
    int k = -1;
    while(j < t_len){
        if(k==-1 || t[j]==t[k]){
            j++;
            k++;
            next[j] = k;
        }
        else
            k = next[k];
    }
}

###############################################
Can anyone tell me WHY???

Edited by author 11.07.2017 19:31

Maybe you don't process characters with negative ASCII correctly.  I don't know how standard string functions deal with negative ASCII.