#include "bits/stdc++.h"
using namespace std;
const int N = 301;
const int inf = 1e9;
int dp[N][N];
int main(){
int n,m;
cin >> n >> m;
string s,s1;
cin >> s;
cin >> s1;
for (int i = 0;i < n;i++){
for (int j = 0;j <= m;j++){
dp[i][j] = inf;
}
}
for (int i = 0;i < n;i++){
dp[i][m] = 0;
}
for (int b = m-1;b >= 0;b--){
for (int a = 0;a < n;a++){
for (int i = 0;i < n;i++){
if (s[i]^s[a])
continue;
for (int j = -1;j < 2;j++){
if (!j)
continue;
if (i+j < 0 || i+j >= n)
continue;
if (s[i+j] ^ s1[b])
continue;
dp[a][b] = min(dp[a][b],dp[i+j][b+1]+abs(i-a)+1);
}
}
}
}
int p=inf;
for (int i = 0;i < n;i++){
if (s[i] == s1[0])
p = min(p,dp[i][1]);
}
if (p >= inf)
p=-1;
cout << p << "\n";
}
