#include <bits/stdc++.h>
using namespace std;
int n, m, dp[510][510] = {0};
string s, t;
int main () {
memset(dp, 63, sizeof(dp));
ios_base::sync_with_stdio(false); cin.tie(0);
cin >> n >> m >> s >> t;
t = "%" + t;
for (int i = 0; i < n; i++) dp[0][i] = 0;
for (int i = 1; i <= m; i++) {
for (int j = 0; j < n; j++) {
if (s[j] != t[i]) continue;
for (int lst = 0; lst < n; lst++) dp[i][j] = min(dp[i][j], dp[i - 1][lst] + abs(j - lst));
}
}
int ans = INT_MAX;
for (int i = 0; i < n; i++) ans = min(ans, dp[m][i]);
if (ans > 100000) ans = -1;
cout << ans << '\n';
}
