#include <bits/stdc++.h>
using namespace std;
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cerr << name << " : " << arg1 << std::endl;
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cerr.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
int main() {
  int n, s;
  cin >> n >> s;
  int ans = s;
  for (int i = 0; i < n; i++) {
    int f, t;
    cin >> f >> t;
    ans = max(ans, f + t);
  }
  cout << ans << endl;
  return 0;
}
