#include <iostream>
#include <stdio.h>
#include <algorithm>

#define NMAX 50003

using namespace std;

int n,m,i,hmax,s,mx;

struct oras{
    int d,l;
}v[NMAX];

int cmp(oras a,oras b){
    return a.d<b.d;
}

int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);

    scanf("%d%d",&m,&n);

    for(i=1;i<=n;i++){
        scanf("%d%d",&v[i].d,&v[i].l);
    }

    sort(v+1,v+n+1,cmp);

    for(i=1;i<=n;i++){
        hmax=max(hmax,mx+v[i].d+v[i].l);
        mx=max(mx,v[i].l-v[i].d);
    }

    printf("%d",hmax);



    return 0;
}