#include<fstream>
#include<algorithm>
using namespace std;
ifstream fi("orase.in");
ofstream fo("orase.out");

const int maxn = 50004;

struct oras{
       int x;
       int y;
};

oras v[maxn];
int d,maxd=0;
int i,n,m;

bool comp(oras a,oras b){
     return (a.x<b.x); 
}

int main(){
    fi>>m>>n;
    for(i=1;i<=n;i++) fi>>v[i].x>>v[i].y;
    
    sort(v+1,v+n+1,comp);
    
    d=v[1].y-v[1].x;
    for(i=2;i<=n;i++){
                      if(d+v[i].x+v[i].y>maxd) maxd=d+v[i].x+v[i].y;
                      if(v[i].y-v[i].x>d) d=v[i].y-v[i].x;
                     }
    fo<<maxd;
    
    fi.close();
    fo.close();
    return 0;
}