o61_oct_c1_charprint

#include <bits/stdc++.h>

using namespace std;
const int inf = 1e9;
const int maxAZ = 26;
const int maxN = 3e5;
vector <int> AZ[maxAZ + 10];
int szAZ[maxAZ + 10], curAZ[maxAZ + 10];
int mark[maxN + 10];

int to_int(char a){
    return a - 'a' + 1;
}

int Sum(int pst, int n, int sum = 0){
    for(int i=pst;i>=1;i-=i&-i)
        sum += mark[i];
    return sum;
}

void Update(int pst, int n){
    for(int i=pst;i<=n;i+=i&-i)
        mark[i] ++;
}

int main(){

    int k;
    scanf("%d", &k);

    string prt;
    cin >> prt;
    int len_prt = prt.size();
    for(int i=0;i<len_prt;i++){
        char p = prt[i];
        AZ[to_int(p)].push_back(i + 1);
        szAZ[to_int(p)] ++;
    }

    string str;
    cin >> str;
    int len_str = str.size();
    long long ans = 0;
    for(auto s: str){
        int ints = to_int(s);
        if(curAZ[to_int(s)] == szAZ[to_int(s)]){
            printf("-1");
            return 0;
        }
        int pst = AZ[ints][curAZ[ints]];
        curAZ[ints] ++;
        ans += pst;
        if(k == 1){
            ans -= (long long) Sum(pst, len_prt);
            Update(pst, len_prt);
        }
    }

    printf("%lld", ans);

    return 0;
}