// Problem: CF1132C Painting the Fence
    // Contest: Luogu
    // URL: https://www.luogu.com.cn/problem/CF1132C
    // Memory Limit: 250 MB
    // Time Limit: 2000 ms
    // 
    // Powered by CP Editor (https://cpeditor.org)
     
#include<bits/stdc++.h>
using namespace std;
struct wall{
	int l,r;
}w[5001];
int cnt[5001];
struct Cf{
	int num,pnt;
}cf[5001];
bool cmp(Cf a,Cf b){
	return a.num<b.num;
}
int main(){
    memset(cnt,0,sizeof(cnt));
    int n,q;
	cin>>n>>q;
    for(int i=1;i<=q;i++){
    	cin>>w[i].l>>w[i].r;
		for(int j=w[i].l;j<=w[i].r;j++){
			cnt[j]++;
		}
	}
	for(int i=1;i<=q;i++){
		cf[i].pnt=i;
		for(int j=w[i].l;j<=w[i].r;j++){
			if(cnt[j]>=1) cf[i].num++; 
		}
	}
	sort(cf+1,cf+q+1,cmp);
	for(int i=1;i<=2;i++){
		for(int j=w[cf[i].pnt].l;j<=w[cf[i].pnt].r;j++){
			cnt[j]--;
		}
		// cout<<cf[i].num<<" ";
	}
	// cout<<"\n";
	int ans=0;
	for(int i=1;i<=n;i++){
		if(cnt[i]) ans++;
	}
	cout<<ans;
	return 0;
}

my output

1784

std's output

1785