#include <iostream>
#include <fstream>
using namespace std;
int main()
{
    ifstream in("orase.in");
    ofstream out("orase.out");
    int m,n,x,i,y,st=0,dr=0;
    in>>m>>n;
    for(i=1;i<=n;i++)
    {
        in>>x>>y;
        if(x+y>st)
            st=x+y;
        if(m-x+y>dr)
            dr=m-x+y;
    }
    out<<st+dr-m;
    return 0;
}