#include<fstream>
#include<vector>
using namespace std;
ifstream in ("orase.in");
ofstream out ("orase.out");
int x,y,n,k,departe,maxim;
vector<int>q[1000001];
int main (void) {
    in >> n >> k;
    for (int i = 1; i <= k; i ++) {
        in >> x >> y;
        q[x].push_back(y);
    }
    for (int i = 0; i <= n; i ++) {
        for (int j = 0; j < q[i].size(); j ++) {
            if (q[i][j] != 0 && i != 0) {
                maxim = max(maxim,departe+q[i][j]);
            }
            if (departe < q[i][j]) {
                departe = q[i][j];
            }
        }
        departe ++;
    }
    out << maxim;
    return 0;
}