#include <fstream>
#include <vector>
#include <map>
#include <cstring>
#include <queue>
#include <algorithm>
using namespace std;

ifstream cin("orase.in");
ofstream cout("orase.out");

int main() {
    int n, m, d, l;
    cin >> m >> n >> d >> l;
    int i = d + l;
    int j = l - d;
    for (int k = 1; k < n; k++) {
        cin >> d >> l;
        if (d + l > i)
            i = d + l;
        if (l - d > j)
            j = l - d;
    }
    cout << i + j;
    return 0;
}