#include <fstream>
#include <algorithm>
#define nmax 50001

using namespace std;

ifstream fin("orase.in");
ofstream fout("orase.out");

int m,n,i,rez,v[nmax];

pair < int , int > a[nmax];

int main()
{
    fin>>n>>m;
    for(i=1;i<=m;i++)
        fin>>a[i].first>>a[i].second;
    sort(a+1,a+m+1);

    for(i=1;i<=m;i++)
    {
        if(a[i].first+a[i].second-a[i-1].first+v[i-1]>rez)
        {
            rez=a[i].first+a[i].second-a[i-1].first+v[i-1];
        }
        v[i]=a[i].second;

    }
    fout<<rez;
    return 0;
}