#include <bits/stdc++.h>
using namespace std;
ifstream in("orase.in");
ofstream out("orase.out");
int main()
{
    int a, b, mx = 0, mx2 = 0;
    in >> a >> a;
    while(in >> a >> b)
        mx = max(mx, a + b), mx2 = max(mx2, b - a);
    out << mx + mx2;
    return 0;
}