#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
using namespace std;
int main() {
  cin.tie(0); cout.tie(0);
  ios::sync_with_stdio(false);
  int N;
  cin >> N;
  vector<int> A(N);
  for(int i = 0; i < N; i++) {
    cin >> A[i];
  }
  if(N == 1) {
    cout << (A[0] <= 1 ? "Yes" : "No") << '\n';
  } else if(N == 2) {
    cout << (A[0] <= 1 || A[1] <= 1 ? "Yes" : "No") << '\n';
  } else {
    vector<bool> ok(N, false);
    for(int i = 0; i < N - 1;) {
      if(A[i] <= 1) {
        ok[i] = true;
        for(int j = i + 1; j < N; j++) {
          i = j;
          if(A[j] >= A[j - 1]) {
            ok[j] = true;
          } else {
            break;
          }
        }
      } else {
        i++;
      }
    }
    for(int i = N - 1; i > 0;) {
      if(A[i] <= 1) {
        ok[i] = true;
        for(int j = i - 1; j >= 0; j--) {
          i = j;
          if(A[j] >= A[j + 1]) {
            ok[j] = true;
          } else {
            break;
          }
        }
      } else {
        i--;
      }
    }
    cout << (accumulate(ok.begin(), ok.end(), 0) == N ? "Yes" : "No") << '\n';
  }
  return 0;
}