#include<stdio.h>
#include<algorithm>
#include<vector>
#include<sstream>
#include<iostream>
#include <iomanip>
#include<cstdlib>
#include<cmath>
#include<cstring>
#define pb push_back
#define mp make_pair
#define fs first
#define sc second
using namespace std;

int N;
vector<pair<int,int> > v;
int ret=-101010,maxim,maxInd,maxaux,M;
int main()
{
    freopen("orase.in","r",stdin);
    freopen("orase.out","w",stdout);
    scanf("%d%d",&M,&N);
    for(int i=1;i<=N;++i){
        int x,y;
        scanf("%d%d",&x,&y);
        v.pb(mp(x,y));
    }
    sort(v.begin(),v.end());
    for(int i=0;i<v.size();++i){
        if(maxim<v[i].fs+v[i].sc){
            maxim = v[i].fs+v[i].sc;
            maxaux=maxim;
            maxInd = i;
        }
    }
    maxaux -=v[0].fs;

    for(int i=0;i<maxInd;++i)
    {
        if(i != 0)
        {
            maxim -= v[i].fs - v[i-1].fs;
        }
        ret = max(ret,maxim+v[i].sc);

    }

    for(int i=0;i<v.size();++i){
        if(maxim<v[i].fs+v[i].sc){
            maxim = M-v[i].fs-v[i].sc;
            maxaux=maxim;
            maxInd = i;
        }
    }
    maxim = maxaux + v[N-1].fs - M;
    for(int i=N-1;i>maxInd;--i){
        if(i!= N-1){
            maxim -= -v[i].fs + v[i+1].fs;
        }

        ret = max(ret,maxim+v[i].sc);
    }
    printf("%d",ret);
    return 0;
}