#include <cstdio>
#include <ctime>
#include <cstdlib>
using namespace std;

FILE *f, *g;

int rez;

unsigned short apa[26 + 1];
unsigned short apb[26 + 1];

int n;
char a[50009 + 2];
char b[50009 + 2];

unsigned short k;
unsigned short lst[26 + 1];
unsigned short nod[50009 + 1];
unsigned short urm[50009 + 1];

unsigned short aib[50009 + 1];

int getLen()
{
    int rez = 1;
    while(a[rez] >= 'a' && a[rez] <= 'z')
        rez ++;

    return rez;
}

void modif(char a[])
{
    int i;
    for(i = n; i >= 1; i --)
        a[i] = a[i - 1];
}

void readFile()
{
    f = fopen("swap.in", "r");

    fgets(a, 50009 + 2, f);
    fgets(b, 50009 + 2, f);

    n = getLen();

    modif(a);
    modif(b);

    fclose(f);
}

int pos()
{
    int i;
    int rez = 1;
    for(i = 1; i <= n; i ++)
    {
        apa[a[i] - 'a' + 1] ++;
        apb[b[i] - 'a' + 1] ++;
    }

    i = 1;
    while(i <= 26 && rez == 1)
    {
        rez = rez && (apa[i] == apb[i]);

        i ++;
    }

    return rez;
}

void addChar(int a, int b)
{
    k ++;

    nod[k] = b;
    urm[k] = lst[a];

    lst[a] = k;
}
void getRez()
{
    int i;
    for(i = 1; i <= n; i ++)
    {
        addChar(a[i] - 'a' + 1, i);
    }

    int p2, s;
    for(i = n; i >= 1; i --)
    {
        int nr = b[i] - 'a' + 1;
        int poz = nod[lst[nr]];
        lst[nr] = urm[lst[nr]];

        int p2 = poz + 1;
        while(p2 <= n)
        {
            aib[p2] += 1;

            p2 += p2 & (-p2);
        }

        s = 0;
        p2 = poz;
        while(p2 > 0)
        {
            s += aib[p2];

            p2 -= p2 & (-p2);
        }
        poz -= s;

        rez += i - poz;
    }
}

void solve()
{
    if(pos() == 0)
    {
        rez = -1;

        return;
    }

    getRez();
}

void printFile()
{
    g = fopen("swap.out", "w");

    fprintf(g, "%d\n", rez);
    fclose(g);
}

int main()
{
    readFile();

    solve();

    printFile();

    return 0;
}