/*
 * Lefter Sergiu - 14/12/2023
 */
#include <fstream>

using namespace std;

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

const int INF = 2e9;

int main() {
    int n, m, sc, smax = -INF;
    in >> n >> m;
    sc = n;
    for (int i = 1; i <= m; ++i) {
        int x, y;
        in >> x >> y;
        sc += x + y;
        if (sc > smax) {
            smax = sc;
        }
        sc = n;
    }
    out << smax;
    in.close();
    out.close();
    return 0;
}