#include <fstream>
#include <cmath>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
int main()
{
    int n,m,i,v[50005],s[50005],maxx=-1,maxx1=-1,k;
    in>>n>>m;
    for (i=1;i<=m;i++)
    {
        in>>v[i]>>s[i];
        if (s[i]>maxx)
        {
            maxx=s[i];
            k=v[i];
        }
    }
    for (i=1;i<=m;i++)
    {
        if (s[i]!=maxx)
        {
            if (s[i]+maxx+abs(k-v[i])>maxx1)
                maxx1=s[i]+maxx+abs(k-v[i]);
        }
    }
    out<<maxx1;
    return 0;
}