#include <iostream>
#include <vector>
using namespace std;

int main() {
  int n, m, ans = 1;
  cin >> n >> m;
  vector<int> a(m);
  for (auto&e : a) cin >> e;
  for (auto it = a.rbegin(); it != a.rend(); it++) {
    if (ans == 1) {
      ans = *it;
    } else {
      if (ans <= *it) ans--;
    }
  }
  cout << ans << endl;
}