#include <bits/stdc++.h>
int a[300004], pos[300004];
using namespace std;
int main() {
  int n, temp;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
    pos[a[i]] = i;
  }
  bool flag = false;
  for (int i = 1; i <= n && !flag; i++)
    for (int j = i + 1; j <= min(n, i + 5) && !flag; j++) {
      temp = 2 * a[j] - a[i];
      if (temp <= n && temp > 0 && pos[temp] > j) flag = 1;
      temp = 2 * a[i] - a[j];
      if (temp <= n && temp > 0 && pos[temp] < i) flag = 1;
    }
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
