class Solution:
    def firstBadVersion(self, n):
        """
        :type n: int
        :rtype: int
        """
        l = 1
        r = n
        while l<r:
            mid = l + (r-l)//2
            if isBadVersion(mid):
                r = mid
            else:
                l = mid+1
        return l if isBadVersion(l) is True else None