#include <fstream>

using namespace std;

const int DMax=1000001, LMax=1000001;

int main()
{
    int m, n, i, distmax, k=2, aux, d[DMax], l[LMax];
    ifstream in("orase.in");
    ofstream out("orase.out");
    in>>m>>n;

    for(i=1; i<=n; i++)
        in>>d[i]>>l[i];

    for(i=1; i<=n; i++)
    {
        if(d[i]>d[i+1])
        {
            aux=d[i];
            d[i]=d[i+1];
            d[i+1]=aux;

            aux=l[i];
            l[i]=l[i+1];
            l[i+1]=aux;

        }
    }

    for(i=1; i<=n; i++)
    {
        if(l[i]+l[k]+d[k]-d[i]>l[i]+l[k+1]+d[k+1]-d[i])
        {
            distmax=l[i]+l[k+1]+d[k+1]-d[i];
            k++;
        }
        else
        {
            distmax=l[k]+l[k+1]+d[k+1]-d[k];
            k++;
        }

    }
    out<<distmax;
    return 0;
}