#include <iostream>
#include <fstream>

using namespace std;
ifstream f("orase.in");
ofstream g("orase.out");
int a[50010],b[50010],i,j,n,m,k,sum=0;
int main()
{
    f>>m>>n;
    for(i=1;i<=n;i++)
        f>>a[i]>>b[i];
    int q;
    for(i=1;i<n;i++)
    for(j=i+1;j<=n;j++)
    {
        q=max(a[i],a[j])-min(a[i],a[j]);
        if(sum<(b[i]+b[j]+q))
            sum=b[i]+b[j]+q;
    }
    g<<sum;
}