#include <bits/stdc++.h>
#define pii pair<int, int>
#define mp make_pair
#define F first
#define S second
#define iter(a) a.begin(), a.end()
using namespace std;
typedef long long ll;
const ll MAX = 2147483647;
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n, m;
cin >> n >> m;
string a, b;
cin >> a >> b;
vector<vector<ll>> dp(m, vector<ll>(n, MAX));
for(int i = 0; i < n; i++){
if(a[i] == b[0]) dp[0][i] = 0;
}
for(int i = 0; i < m - 1; i++){
for(int j = 0; j < n; j++){
if(dp[i][j] == MAX) continue;
for(int k = 0; k < n; k++){
if(a[k] == b[i]) dp[i][k] = min(dp[i][k], dp[i][j] + abs(j - k));
}
}
for(int j = 0; j < n; j++){
if(dp[i][j] == MAX) continue;
if(j && a[j - 1] == b[i + 1]) dp[i + 1][j - 1] = min(dp[i + 1][j - 1], dp[i][j] + 1);
if(j < n - 1 && a[j + 1] == b[i + 1]) dp[i + 1][j + 1] = min(dp[i + 1][j + 1], dp[i][j] + 1);
}
}
int ans = *min_element(iter(dp[m - 1]));
if(ans == MAX) ans = -1;
cout << ans << "\n";
return 0;
}
