附上代码

#include<bits/stdc++.h>
using namespace std;
int l,m;
struct d{
	int u,v;
}doge[101];
bool cmp(d x,d y)
{
	if(x.u>y.u)
	{
		return x.u<y.u;
	}
}
int main()
{
	cin>>l>>m;
	l++;
	for(int i=0;i<m;i++)
	{
		cin>>doge[i].u>>doge[i].v;
	}
	sort(doge,doge+m,cmp);
	for(int i=1;i<m;i++)
	{
		if(doge[i].u<doge[i-1].v)//去重合部分
		{
			doge[i].u=doge[i-1].v+1;
		}
		if(doge[i].v<doge[i].u)//如果全重合就删去
		{
			doge[i].u=0;
			doge[i].v=0;
		}
	}
	for(int i=0;i<m;i++)
	{
		if(doge[i].u==0 and doge[i].v==0)
		{
			continue;
		}
		l=l-(doge[i].v-doge[i].u+1);
	}
	cout<<l;
	return 0;
}