#include <stdio.h>

// rad[i] = max radius of palindrome with center between s[i-1] and s[i]
void Manacher_even(int N, char s[], int rad[])
{
	int i, j, l, r;
	for (i = 1, rad[0] = 0, rad[1] = 0, rad[N] = 0; i < N; i = r) {
		for (l = i - rad[i] - 1, r = i + rad[i]; l >= 0 && r < N && s[l] == s[r]; l--, r++);
		rad[i] = r - i;
		for (l = i - 1, r = i + 1; r - i < rad[i] && l > 0 && r < N; l--, r++) {
			if (l - rad[l] == i - rad[i]) {
				rad[r] = i + rad[i] - r;
				break;
			} else if (l - rad[l] > i - rad[i]) rad[r] = rad[l];
			else rad[r] = i + rad[i] - r;
		}
	}
}

void max_palindrome_naive(int N, char s[], int rad[])
{
	int i, l, r;
	for (i = 1, rad[0] = 0, rad[N] = 0; i < N; i++) {
		for (l = i - 1, r = i; l >= 0 && r < N && s[l] == s[r]; l--, r++);
		rad[i] = r - i;
	}
}

int main()
{
	int i, j, N, M, rad[500001];
	char S[500001], T[500001];
	scanf("%d %d", &N, &M);
	scanf("%s", S);
	scanf("%s", T);
	Manacher_even(M, T, rad);
	if (M % 2 != 0 || rad[M/2] != M / 2) {
		printf("-1\n");
		fflush(stdout);
		return 0;
	}
	
	int dist[500001] = {}, q[2000001], head = 1000000, tail = 1000000;
	for (i = 0; i < N && i < M && S[i] == T[i]; i++);
	for (j = 0; j < N && j < M && S[N-j-1] == T[M-j-1]; j++);
	if (i < j) i = j;
	if (i == M) i--;
	dist[i] = 1;
	q[tail++] = i;
	for (; head < tail; head++) {
		i = q[head];
		if (i > 0 && (dist[i-1] == 0 || dist[i-1] > dist[i])) {
			dist[i-1] = dist[i];
			q[head--] = i - 1;
		}
		j = i + rad[i];
		if (dist[j] == 0) {
			dist[j] = dist[i] + 1;
			q[tail++] = j;
		}
	}
	printf("%d\n", dist[M] - 1);
	fflush(stdout);
	return 0;
}