#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
struct punct{
    int x,y;
};
punct v[50007];
bool cmp(punct a,punct b){
    return a.x<b.x;
}
int main()
{
    int i,m,n,dc,dmax=0,a,b,j,u;
    f>>m>>n;
    for(i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    sort(&v[1],&v[n+1],cmp);
    u=1;
    for(i=2;i<=n;i++)
    {
        dc=v[u].y+v[i].y+(v[i].x-v[u].x);
        if(dc>dmax) dmax = dc;
        u=i;
        if(v[i].y>v[u].y+v[i].x-v[u].x)
            u=i;
    }
    g<<dmax;
    return 0;
}