simple num by choose

static long GetNumByChoose(int[] choose, int n, int k)
{
    long result = 0;
    int lastUsedNumber = 0;
    for (int pos = 0; pos < k; pos++)
    {
        for (int i = lastUsedNumber+1; i < choose[pos]; i++)
            result += C(n-i, k - pos - 1);
        lastUsedNumber = choose[pos];
    }
    return result;
}