#include <fstream>
using namespace std;
ifstream cin("orase.in");
ofstream cout("orase.out");
int main(){
    int length, n, small_distance=0, big_distance=-1;
    cin>>length>>n;
    for(int i=0; i<n; i++){
        int distance, height;
        cin>>distance>>height;
        small_distance = max(small_distance, height - distance);
        big_distance = max(big_distance, distance + height);
    }
    cout<<small_distance + big_distance;
}