#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;
long long 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());
    maxim = v[0].sc;
    for(int i=1;i<v.size();++i){

        maxim += v[i].fs - v[i-1].fs;
        if(maxim+v[i].sc > ret){
            ret=maxim+v[i].sc;
        }
        if(maxim < v[i].sc)
        {
            maxim = v[i].sc;
        }
        //printf("%lld ",ret);
    }
    printf("%lld",ret);
    return 0;
}