#include <bits/stdc++.h>
using namespace std;
const int N = 310;
int dp[N][N];
vector<int> idxes[30];
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;
for(int i = 1; i <= n; ++i) idxes[a[i]-'a'].push_back(i);
memset(dp, 0x3f, sizeof dp);
for(int i : idxes[b.front()-'a']) dp[1][i] = 0;
b = '.'+b;
for(int i = 2; i <= m; ++i){
for(int j : idxes[b[i]-'a']){
int mn = 1e9;
for(int k : idxes[b[i-1]-'a']){
if(j == k) continue;
if(abs(j-k) < abs(j-mn)) mn = k;
dp[i][j] = min(dp[i][j], dp[i-1][k] + abs(j-k));
}
if(mn == 1e9) continue;
for(int k : idxes[b[i-1]-'a']){
dp[i][j] = min(dp[i][j], dp[i][k] + abs(j-mn) + 1);
}
}
}
int res = 1e9;
for(int i = 1; i <= n; ++i) res = min(res, dp[m][i]);
cout << (res == 1e9 ? -1 : res) << '\n';
}
