#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <algorithm>
#define N 1000100

using namespace std;

struct dr{
    int d,l;
} drum[N];;
bool cmp(struct dr x,struct dr y){
    return x.d<y.d;
}
int main(){
    int i,m,n,Dmax,Dc,Max=0;

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

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

    for(i=0;i<n;i++){
        scanf("%d%d",&drum[i].d,&drum[i].l);
    }

    sort(drum,drum+n,cmp);

    Dmax=0;
    for(i=0;i<n-1;i++){
        Dc=drum[i].l-drum[i].d;
        if(Dc>Dmax){
            Dmax=Dc;
        }
        if(drum[i+1].d+drum[i+1].l+Dmax>Max){
            Max=drum[i+1].d+drum[i+1].l+Dmax;
        }
    }

    printf("%d",Max);

    return 0;
}