bool reachable_in( unsigned n, char x1, char y1, char x2, char y2 )
{
    if ( x1 == x2 && y1 == y2 )
        return true;
    if ( n == 0 || x1 >= 8 || x1 < 0 || y1 >= 8 || y1 < 0 )
        return false;

    for ( int dx = 1; dx <= 2; ++dx )
    {
        int dy = 3 - dx;
        for ( int sx = -1; sx < 2; sx += 2 )
        {
            for ( int sy = -1; sy < 2; sy += 2 )
            {
                if ( reachable_in( n - 1, x1 + sx * dx, y1 + sy * dy, x2, y2 ) )
                    return true;
            }
        }
    }
    return false;
}

unsigned knight_hops( char x1, char y1, char x2, char y2 )
{
    int n = 0;

    while( !reachable_in( n, x1, y1, x2, y2 ) )
        ++ n;

    return n;
}