#include<bits/stdc++.h>
#define int long long
#define IOS ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define awa return 0;
using namespace std;
int n,m;
string s,t;
bool find(string s,string t)
{
    unordered_map<char,int>scnt;
    for(char c:s)
    {
        scnt[c]++;
    }
    unordered_map<char,int>tcnt;
    for(char c:t)
    {
        tcnt[c]++;
        if(scnt[c]<tcnt[c])
        {
            return false;
        }
    }
    return true;
}
signed main()
{
    IOS;
    cin>>n>>m;
    cin>>s>>t;
    if(find(s,t))
    {
        
    }
    else
    {
        cout<<-1;
        awa
    }
    awa
}
//