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

#define NMAX 50003

using namespace std;

int n,m,i,hmax,s,x,y,dif;

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++){
        if(hmax<v[i].l-v[i].d){
            hmax=v[i].l-v[i].d;
            x=v[i].l;
            y=v[i].d;
        }

    }

    dif=(v[n].d-y)+v[n].l+x;

    printf("%d",dif);



    return 0;
}