#include <fstream>
#include <algorithm>
using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");

struct oras
{
    int x,y;
};
oras v[50002];

bool cmp(oras a,oras b)
{
    return a.x<b.x;
}

int main()
{
    int m,n,i,max=-100,j,u=1;
    cin>>m;
    cin>>n;
    for(i=1; i<=n; i++)
        cin>>v[i].x>>v[i].y;
    sort(v+1,v+n+1,cmp);
    for(i=2; i<=n; i++)
    {
        if(v[i-1].y>v[u].y+v[i-1].x-v[u].x)
            u=i-1;
        if (v[u].y - v[u].x + v[i].x + v[i].y > max)
            max = v[u].y - v[u].x + v[i].x + v[i].y;
    }
    cout<<max;
    cin.close();
    cout.close();
    return 0;
}