#include<bits/stdc++.h>
using namespace std;
long long n, m, mini = INT_MAX;
string s, str;
vector<int> vec[30];
long long backtrack(int pos, int x)
{
if(x == m) return 0;
long long temp = INT_MAX;
for(int i : vec[s[pos] - 'a'])
{
if(i < n - 1 && s[i + 1] == str[x]) temp = min(temp, backtrack(i + 1, x + 1) + abs(pos - i) + 1);
if(i > 0 && s[i - 1] == str[x]) temp = min(temp, backtrack(i - 1, x + 1) + abs(pos - i) + 1);
}
return temp;
}
int main()
{
cin >> n >> m >> s >> str;
for(int i = 0; i < n; ++i) vec[s[i] - 'a'].push_back(i);
for(int c : vec[str[0] - 'a']) mini = min(mini, backtrack(c, 1));
if(mini < INT_MAX) cout << mini;
else cout << -1;
}
