#include <cstdio>
#include<algorithm>
#include<vector>
#include<cmath>
using namespace std;

FILE *f=fopen("orase.in", "r");
FILE *g=fopen("orase.out", "w");

struct date{
    int d, l;
};
vector <date> v;

bool cmp(date a, date b){
    return (a.d<b.d);
}

int main()
{
    int n, maxx=-500, maxi=-500, m;
    date aux, bux;
    fscanf(f, "%d%d", &m, &n);
    for(int i=1; i<=n; ++i){
        fscanf(f, "%d%d", &aux.d, &aux.l);
        v.push_back(aux);
    }
    sort(v.begin(), v.end(), cmp);
    aux=v[0];
    maxx=abs(aux.l-aux.d);
    for(int i=1; i<n; ++i){
        aux=v[i];
        maxi=max(maxi, maxx+aux.d+aux.l);
        maxx=max(maxx, abs(aux.d-aux.l));
    }
    fprintf(g, "%d", maxi);
return 0;
}