#include<iostream>
#include<fstream>
#include<algorithm>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
struct oras{
    int x,y;
};
const int ff=51000;
bool cmp(oras a,oras b){
    return a.x<b.x;
}
oras v[ff];
int d(int u,int i){
    return v[i].y+v[u].y+v[i].x-v[u].x;
}
int main()
{
    int m,n,i,u=1,aux,max=0;
    in>>m>>n;
    for(i=1;i<=n;i++){
        in>>v[i].x>>v[i].y;
    }
    sort(&v[1],&v[n+1],cmp);
    u = 1;
    for(i=2;i<=n;i++){
        if(d(u,i)>max) max = d(u,i);
        if(v[i].y > v[u].y + v[i].x - v[u].x) u = i;
    }
    out<<max;
    return 0;
}