#define N 10000

void mpMul(int *ret, int *a, int *b)
{
    int  i, j;
    int  la, lb;
    int  *aa;
    int  ca;
    long x;

    la = *a;
    lb = *b;

    if (la == 0 || lb == 0) {
        *ret = 0;
        return;
    }

    for (i = la + lb; i > 0; i--) *(ret + i) = 0;

    for (j = 1; j <= lb; j++) {
        ca = 0;
        b++;
        for (i = 1, aa = a; i <= la; i++) {
            x = *++aa;
            x = x * *b + *(ret + i + j - 1) + ca;
            *(ret + i + j - 1) = x % N;
            ca = x / N;
        }
        *(ret + i + j - 1) = ca;
    }
    
    *ret = (ca != 0) ? la + lb : la + lb - 1;
}