#include <iostream>
#include <fstream>

using namespace std;

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

int main() {
	int M, N, D, L;
	fin >> M >> N;
	int sum = M;
	int sumMax = 0;
	for (int i = 0; i < N; ++i) {
		fin >> D >> L;
		sum += D + L;
		if (sumMax < sum) {
			sumMax = sum;
		}
		sum = M;
	}
	fout << sumMax;
	fin.close(); fout.close();
	return 0;
}