public int findFirstBadVersion(int n) {
        int left = 1;
        int right = n;
        while(left + 1< right){
            int mid = left + (right - left)/2;
            if(SVNRepo.isBadVersion(mid)){
              right = mid;    
            }
            else{
                left = mid +1;
            }
        }
        if(SVNRepo.isBadVersion(left)){
            return left;
        }
        return right;
    }