#include<bits/stdc++.h>

using namespace std ;
typedef long long ll ;
const int MX = 500500 ;
char input1[MX] , input2[MX] ;
string a , b ;
int l ;
bool rev( int i )
{
    return ( (a[l-1-i] == b[i]) && ( a[i] == b[l-i-1] ) ) ;
}
int main()
{
    int t ;
    scanf("%d" , &t) ;
    while( t-- )
    {
        scanf("%s" , input1) ;
        a = (string)input1 ;
        scanf("%s" , input2) ;
        b = (string)input2 ;
        l = a.length() ;
        if( a.length() != b.length() )
        {
            printf("-1\n") ;
            continue ;
        }
        if( a[l/2] != b[l/2] || ( l == 1 && ( a != b ) )  ){
            printf("-1\n") ;
            continue ;
        }
        int pr = 0 , ans = 0 ;
        for( int i = 0 ; i < l / 2 ; ++i )
        {
            if( pr )
                swap( a[i] , a[l-1-i] ) ;
            if( (a[i] == b[i]) && (a[l-1-i] == b[l-1-i]) )continue ;

            else if( rev(i) )
            {
                ans++ ;
                pr = 1 - pr ;
            }
            else
            {
                ans = -1 ;
                break ;
            }
        }
        printf("%d\n",ans) ;
    }
    return 0 ;
}