#include <fstream>
using namespace std;
ifstream cin ("orase.in");
ofstream cout ("orase.out");
int main() {
    int n,max1=0,max2=0,a,b,m;
    cin>>n>>m;
    for (int i=0; i<m; i++) {
        cin >> a >> b;
        if(b+a>max1) max1=b+a;
        if(b-a>max2) max2=b-a;
    }
    cout<<max1+max2;
}