#include <algorithm>#include <cstring>#include <stdio.h>
#include <vector>
using namespace std;
char A[ 50001 ], B[ 50001 ];
int N, XXX, F[ 50001 ], P[ 50001 ];
int sum, arb[ 50001<<2 ];
vector <int> poz[ 26 ];

inline void query( const int &nod, const int &st, const int &dr, const int &a, const int &b ) {

    int mij;

    if( a <= st && dr <= b ) {

        sum += arb[ nod ];
        return;
    }
    mij = (st+dr) / 2;
    if( a <= mij )
        query( nod<<1, st, mij, a, b );
    if( mij < b )
        query( (nod<<1) + 1, mij+1, dr, a, b );
}

inline void update( const int &nod, const int &st, const int &dr, const int &poz ) {

    int mij;

    if( st == dr ) {

        arb[ nod ] = 1;
        return;
    }
    mij = (st+dr) / 2;
    if( poz <= mij )
        update( nod<<1, st, mij, poz );
    else
        update( (nod<<1) + 1, mij+1, dr, poz );
    arb[ nod ] = arb[ nod<<1 ] + arb[ (nod<<1) + 1 ];
}

int main() {

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

    int i, ind;

    fgets( A+1, 50001, stdin);
    fgets( B+1, 50001, stdin);

    N = strlen( A+1 );
    for( i = 1; i <= N; ++i ) {

        ++F[ A[ i ] - 'a' ];
        --F[ B[ i ] - 'a' ];
    }
    for( i = 0; i < 26; ++i )
        if( F[ i ] ) {

            printf( "-1" );
            return 0;
        }
    for( i = 1; i <= N; ++i ) {

        ++F[ B[ i ] - 'a' ];
        poz[ B[ i ] - 'a' ].push_back( i );
    }
    memset( F, 0, sizeof( F ) );
    for( i = 1; i <= N; ++i ) {

        ind = A[ i ] - 'a';
        ++F[ ind ];
        P[ i ] = poz[ ind ][ F[ ind ] - 1 ];
    }
    memset( F, 0, sizeof( F ) );
    for( i = 1; i <= N; ++i ) {

        update( 1, 1, N, P[ i ] );
        sum = 0;
        query( 1, 1, N, 1, P[ i ] );
        XXX += P[ i ] - sum;
    }

    printf( "%d", XXX );

    return 0;
}