#include <bits/stdc++.h>
using namespace std;
const int N = 310;
int dp[N][N];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int n, m;
cin >> n >> m;
string a, b;
cin >> a >> b;
a = '.'+a;
b = '.'+b;
for(int i = 1; i <= n; ++i) if(a[i] == b[1]) dp[1][i] = 0; else dp[1][i] = 1e9;
int res = 1e9;
for(int i = 2; i <= m; ++i){
for(int j = 1; j <= n; ++j) for(int k = 1; k <= n; ++k) dp[i-1][k] = min(dp[i-1][k], dp[i-1][j]+abs(j-k));
for(int j = 1; j <= n; ++j){
dp[i][j] = 1e9;
if(a[j] == b[i]){
if(j > 1) dp[i][j] = min(dp[i][j], dp[i-1][j-1]+1);
if(j < n) dp[i][j] = min(dp[i][j], dp[i-1][j+1]+1);
}
}
}
for(int i = 1; i <= n; ++i) res = min(res, dp[m][i]);
cout << (res >= 1e9 ? -1 : res) << '\n';
}
