#include <stdio.h>
#include <string.h>
#include <assert.h>


char s[50100], sfin[50100];
int ind[26], perm[50100], AIB[50100];

inline int lsb(int X)
{
    return X & -X;
}

int query(int x)
{
    int cnt = 0;

    while (x)
    {
        cnt += AIB[x];
        x = x - lsb(x);
    }

    return cnt;
}

void update(int x, int N)
{
    while (x <= N)
    {
        AIB[x] ++;
        x += lsb(x);
    }
}

int main()
{
    int i, N;

    freopen("swap.in", "r", stdin);
    freopen("swap.out", "w", stdout);

    fgets(s , 50001, stdin); fgets(sfin, 50001, stdin);
    N = strlen(s + 1);

    for (i = 1; i <= N; i ++)
    {
        int now = s[i] - 'a';
        while (ind[now] <= N)
        {
            ++ ind[now];
            if (sfin[ind[now]] == s[i])
                break;
        }
        if (ind[now] == N + 1)
        {
            printf("-1");
            return 0;
        }
        perm[i] = ind[now];
    }

    int sol = 0;
    for (i = 1; i <= N; i ++)
    {
        sol += i - 1 - query(perm[i]);
        update(perm[i], N);
    }

    printf("%d", sol);
    return 0;
}