#include <bits/stdc++.h>
using namespace std;
const int MAXN = 300000 + 10;
int n, a[MAXN], p[MAXN];
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> a[i], p[a[i]] = i;
  for (int i = 0; i < n; i++) {
    for (int j = min(i + 100, n - 1); j > i; j--) {
      int nx = 2 * a[j] - a[i], nxt = 2 * a[i] - a[j];
      if (nx > 0 && nx <= n && p[nx] > j) return cout << "YES" << endl, 0;
      if (nxt > 0 && nxt <= n && p[nxt] < i) return cout << "YES" << endl, 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
