#include <iostream>
#include <fstream>
using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int main()
{
    int m,n,a,b,c,d,sc=0,smax=-1,dist;
    f>>m>>n;
    f>>a>>b;
    n--;
    while(n!=0)
    {
        f>>c>>d;
        if(a>c)
        {
            dist=a-c;
        }else dist=c-a;
        sc=b+d+dist;
        if(sc>smax)
        {
            smax=sc;
        }
        c=a;
        d=b;
        n--;
    }
    g<<smax;
    return 0;
}