#include <iostream>
#include<fstream>

using namespace std;
    ifstream f("orase.in");
    ofstream g("orase.out");

    int n, m, d, inaltime, maxim, maxim2, poz, poz1;

int main()
{   f>>n>>m;
for(int i=1; i<=m; i++)
    {f>>d>>inaltime;
    if(inaltime>maxim) {if(maxim>maxim2) maxim2=maxim, poz1=poz; maxim=inaltime, poz=d;}
    else if(inaltime>maxim2 and inaltime!=maxim) {maxim2=inaltime, poz1=d;} }
    if(poz>poz1) g<<maxim+maxim2+(poz-poz1);
    else g<<maxim+maxim2+(poz1-poz);
        return 0;
}