#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, s;
  while (cin >> n >> s) {
    int ans = s;
    for (int i = 0; i < n; ++i) {
      int f, t;
      cin >> f >> t;
      if (ans < f + t) {
        ans = f + t;
      }
    }
    cout << ans << endl;
  }
  return 0;
}
