#include <iostream>
#include <fstream>
#include <algorithm>
#define NMAX 50005
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int m,n,mn,mx;
pair<int,int> v[NMAX];
int S[NMAX];
int main()
{
    fin>>m>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].first>>v[i].second;
    sort(v+1,v+n+1);
    for(int i=1;i<=n;i++)
        S[i]=S[i-1]+v[i].first+v[i].second;
    for(int i=1;i<=n;i++)
    {
        if(S[i]-mn-S[i-1]>mx)
            mx=S[i]-mn;
        if(S[i]-v[i].first-S[i-1]<mn)
            mn=S[i];
    }
    fout<<mx<<endl;
    return 0;
}