#include <iostream>
#include <stdio.h>
#include <vector>
#include <algorithm>
using namespace std;
vector<pair<int,int> > V;

int Max,aux;
int N,M,D,L;

int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&M,&N);
    for(int i=1;i<=N;i++)
    {
        scanf("%d%d",&D,&L);
        V.push_back(make_pair(D,L));
    }

    sort(V.begin(),V.end());

    for(auto it:V)
    {
        Max=max(Max,it.first+it.second+aux);
        aux=max(aux,it.second-it.first);
    }

    printf("%d",Max);
}