#include <fstream>
#include <algorithm>
#define pii pair<int,int>
#define Nmax 50005
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
pii v[Nmax];
int main()
{
    int n,m,i,maxx=-1,maxx1=-1;
    in>>n>>m;
    for (i=1;i<=m;i++)
    {
        in>>v[i].first>>v[i].second;
    }
    sort(v+1,v+m+1);
    maxx=v[m].first+v[m].second;
    for (i=m-1;i>=1;i--)
    {
        maxx1=max(maxx1,maxx-v[i].first+v[i].second);
        maxx=max(maxx,v[i].first+v[i].second);
    }
    out<<maxx1;
    return 0;
}