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