#include <fstream>
#include <algorithm>
#define pii pair<int,int>
#define Nmax 50005
using namespace std;

ifstream f("orase.in");
ofstream g("orase.out");

int n,mx,m,res;
pii v[Nmax];

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

    mx = v[n].first+v[n].second;
    for (int i=n-1;i>=1;i--){
        res = max(res,mx-v[i].first+v[i].second);
        mx = max(mx,v[i].first+v[i].second);
    }

    g<<res;
    return 0;
}