#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;
ifstream ka("orase.in");
ofstream ki("orase.out");
int m,n;
unsigned long long dif,maxim;
int abs(int t)
{
    if(t>=0)return t;
    return -t;
}
struct oras
{
    int d,l,r;
}a[50005];
bool comp(oras a,oras b)
{
    return a.r<b.r;
}
int main()
{
    ka>>m>>n;
    for(int i=1;i<=n;i++)
    {
        ka>>a[i].d>>a[i].l;
        a[i].r=a[i].d+2*a[i].l+m-a[i].d;
    }
    sort(a+1,a+n+1,comp);
    ki<<a[n].l+a[n-1].l+abs(a[n].d-a[n-1].d);

}