#include<iostream>
#include<fstream>
using namespace std;
int main()
{
    ifstream in("orase.in");
    ofstream out("orase.out");
    int m,n,v[50001],d[50001],smax=-1,s=0;
    in>>m>>n;
    for(int i=1;i<=n;i++)
        in>>v[i]>>d[i];
    for(int i=1;i<=n-1;i++)
    {
        for(int j=i+1;j<=n;j++)
        {
            if(v[i]>=v[j])
                s=d[i]+d[j]+(v[i]-v[j]);
            else
                s=d[i]+d[j]+(v[j]-v[i]);
            if(s>smax)
                smax=s;
            s=0;
        }
    }
    out<<smax;
    in.close();
    out.close();
    return 0;
}