#include <bits/stdc++.h>
using namespace std;
int n,m,Min=2147483647;
string s,c;
void dfs(int deep,int x,int se);
int main()
{
    scanf("%d %d",&n,&m);
    cin>>s;
    cin>>c;
    for(int i=0;i<n;i++){
        if(s[i]==c[0]){
            dfs(2,i,0);

        }
    }
    if(Min==2147483647){
        puts("-1");
    }else{
        printf("%d\n",Min);
    }
    return 0;
}
void dfs(int deep,int x,int se){
    if(deep>m){
        if(se<Min){
            Min=se;
        }
        return;
    }
    if(x>=1&&s[x-1]==c[deep-1]){
        dfs(deep+1,x-1,se+1);
    }
    if(x<n-1&&s[x+1]==c[deep-1]){
        dfs(deep+1,x+1,se+1);
    }
    for(int i=0;i<n;i++){
        if(i!=x&&s[i]==s[x]){

            int sj=abs(x-i);
            if(i>=1&&s[i-1]==c[deep-1]){
                dfs(deep+1,i-1,se+1+sj);
            }
            if(i<n-1&&s[i+1]==c[deep-1]){
                dfs(deep+1,i+1,se+1+sj);
            }
        }
    }
}