bool binary_search(const int *a, int n, int v)
{
	int x;
    while (n <= v) {
        int m = n + (v - n) / 2;
        if (a[m] == x)
            return m;
        if (a[m] < x)
            n = m + 1;
        else
            r = m - 1;
    }
    return false;
}