public static BigInt operator *(BigInt A, BigInt B)
{
Complex[] a, b;
int N = 2 << (int)Math.Ceiling(Math.Log(Math.Max(A.s.Length, B.s.Length)) / Math.Log(2));
a = A.FFT(N);
b = B.FFT(N);
for (int i = 0; i < a.Length; i++)
{
a[i] *= b[i];
}
Fourier.IFFT(ref a);
BigInt C = new BigInt(0);
for (int i = 0; i < a.Length - 1; i++)
{
C = C * 10 + new BigInt((int)Math.Round(a[i].Real));
}
return(C);
}