这里是代码：

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

class line {
public:
	long long begin;
	long long end;
	bool operator < (const line& other) const {
		return begin < other.begin;
	}
	bool if_one(const line& x, const line& y);  
	void hebing(line& x, line& y);
};

bool line::if_one(const line& x, const line& y)  
{
	if (x.end >= y.begin)
		return true;
	else
		return false;
}

void line::hebing(line& x, line& y)
{
	if (if_one(x, y))
	{
		x.end = max(y.end, x.end);
		y.begin = 0;
		y.end = 0;
	}
}

line lines[20000];  

int main()
{
	int n;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> lines[i].begin;  
		cin >> lines[i].end;  
	}
	sort(lines, lines + n);
	for (int i = 0; i < n - 1; i++)
	{
		if (lines[i].begin < lines[i].end)
		{
			lines[i].hebing(lines[i], lines[i + 1]);
			swap(lines[i], lines[i + 1]);
		}
	}
	long long length = 0;
	for (int i = 0; i < n; i++)
	{
		length += lines[i].end - lines[i].begin;
	}
	cout << length;
	return 0;
}