#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("orase.in");
ofstream fout("orase.out");
int main()
{
    int n,m,a[100000],b[100000],di,li,i,smax=0;
    fin>>m>>n;
    for(i=1;i<=n;i++)
    {
        fin>>a[i]>>b[i];
        if(smax<=a[i]+b[i])
        {
            di=a[i];
            li=b[i];
            smax=a[i]+b[i];
        }
    }
    smax=0;
    for(i=1;i<=n;i++)
    {
        if(di-a[i]+li+b[i]>smax)
            smax=di-a[i]+li+b[i];
    }
    fout<<smax;
    return 0;
}