bs

//true true true true true true true | false false false false
long long MAX_TRUE(bool check(int), long long left, long long right)
{
    long long i = left, j = right, mid;

    while (i<j)
    {
        mid  = (i + j)/2;
        if (check (mid))
            i = mid+1;
        else
            j = mid;
    }
    if (check(i))
        return i;  //all true
    else if (i==left)
        return -1; //all false
    else
        return i-1;
}