Farmer John has decided to assemble a panoramic photo of a lineup of his cows, which, as always, are conveniently numbered from . Accordingly, he snapped photos, each covering a contiguous range of cows: photo i contains cows through inclusive. The photos collectively may not necessarily cover every single cow. After taking his photos, FJ notices a very interesting phenomenon: each photo he took contains exactly one cow with spots! FJ was aware that he had some number of spotted cows in his herd, but he had never actually counted them. Based on his photos, please determine the maximum possible number of spotted cows that could exist in his herd. Output -1 if there is no possible assignment of spots to cows consistent with FJ's photographic results.
给你一个 长度的数轴和 个区间，每个区间里有且仅有一个点，问能有多少个点。
输入格式
Line : Two integers and .
Lines : Line contains and .
输出格式
Line : The maximum possible number of spotted cows on FJ's farm, or -1 if there is no possible solution.
样例
样例输入 #1
5 3
1 4
2 5
3 4
样例输出 #1
1
样例说明
There are cows and photos. The first photo contains cows through , etc. From the last photo, we know that either cow or cow must be spotted. By choosing either of these, we satisfy the first two photos as well.
