#include<bits/stdc++.h>
using namespace std;
int main()
{
	int as,bs;
	string a,b;
	cin>>as>>bs>>a>>b;
	for(int i=0;i<bs;i++)
	{
		if(a.find(b[i])==a.npos)
		{
			cout<<-1;
			return 0;
		}
	}
	int c=a.find(b[0]);cout<<b.size();
	return 0;
}