# ZeroJudge - f855: 線段覆蓋長度 加強版
### 題目連結：https://zerojudge.tw/ShowProblem?problemid=f855
###### tags: `ZeroJudge` `排序（Sorting）` `掃描線演算法（Sweep Line Algorithm）`

#include <iostream>
#include <algorithm>
using namespace std;

struct line {
	int left, right;

	bool operator<(const line &other) const {
		if (this->left != other.left)
			return this->left < other.left;
		return this->right < other.right;
	}

	void operator=(const line &other) {
		this->left = other.left;
		this->right = other.right;
	}
} lines[10000], buffer;

int main() {
	cin.sync_with_stdio(false); cin.tie(nullptr);
	int number, length;
	while (cin >> number) {
		for (int i = 0; i < number; ++i)
			cin >> lines[i].left >> lines[i].right;
		sort(lines, lines + number);
		buffer = lines[0];
		length = 0;
		for (int i = 1; i < number; ++i) {
			if (lines[i].left <= buffer.right)
				buffer.right = max(lines[i].right, buffer.right);
			else {
				length += buffer.right - buffer.left;
				buffer = lines[i];
			}
		}
		cout << (length + buffer.right - buffer.left) << '\n';
	}
}