karatsuba

static long karatsubaMethod(long x,long y)
        {
            long a, b, c, d,ac,bd;
            int len = (int) Math.Floor(Math.Log10(x)+1);
            len /= 2;
            if (len <= 1)
                return x * y;

            a = x / (long)Math.Floor(Math.Pow(10, len));
            b = x % (long)Math.Floor(Math.Pow(10, len));
            c = y / (long)Math.Floor(Math.Pow(10, len));
            d = y % (long)Math.Floor(Math.Pow(10, len));
            ac = karatsubaMethod(a, c);
            bd = karatsubaMethod(b, d);
            return ac * (long)Math.Floor(Math.Pow(10, 2*len))
                +(ac+bd-karatsubaMethod(a-b,c-d))*(long)Math.Floor(Math.Pow(10, len))
                + bd;
        }