// The API isBadVersion is defined for you.
// bool isBadVersion(int version);

class Solution {
public:
    int firstBadVersion(int n) {
        int s=0, e=n-1;
        int mid=s+(e-s)/2;
        int ans=-1;
        while(s<=e)
        {
            if(isBadVersion(mid)) 
            {
                ans=mid;
                e=mid-1;
            }
            else s=mid+1;
            mid=s+(e-s)/2;
        }
        return mid;
    }
};