#include <bits/stdc++.h>

using namespace std;

ifstream fin("swap.in");
ofstream fout("swap.out");

int N;
vector < int > BIT;

void update(int index) {
    for(int i = index; i <= N; i += i & -i)
        ++BIT[i];
}

int query(int index) {
    int ans = 0;
    for(int i = index; i > 0; i -= i & -i)
        ans += BIT[i];
    return ans;
}

int main() {
    string s, t;
    fin >> s >> t;
    vector < int > fs(26), ft(26);
    for(char x : s)
        ++fs[x - 'a'];
    for(char x : t)
        ++ft[x - 'a'];
    bool ok = true;
    for(int i = 0; i < 26 && ok; ++i)
        if(fs[i] != ft[i])
            ok = false;
    if(!ok) {
        fout << -1;
        return 0;
    }
    vector < int > from_s[26], from_t[26];
    N = s.size();
    s = '0' + s, t = '0' + t;
    for(int i = 1; i <= N; ++i)
        from_s[s[i] - 'a'].emplace_back(i);
    for(int i = 1; i <= N; ++i)
        from_t[t[i] - 'a'].emplace_back(i);
    vector < int > perm(N + 1);
    for(int i = 0; i < 26; ++i)
        for(int j = 0; j < (int)from_t[i].size(); ++j) {
            int poz = from_t[i][j];
            perm[poz] = from_s[i][j];
        }
    int ans = 0;
    BIT.resize(N + 1);
    for(int i = N; i > 0; --i) {
        ans += query(perm[i]);
        update(perm[i]);
    }
    fout << ans;
}