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