#include <bits/stdc++.h>
using namespace std;
template <class T>
void read(T &x) {
  x = 0;
  int f = 0;
  char ch = getchar();
  while (ch < '0' || ch > '9') {
    f |= (ch == '-');
    ch = getchar();
  }
  while (ch <= '9' && ch >= '0') {
    x = (x << 1) + (x << 3) + (ch ^ 48);
    ch = getchar();
  }
  x = f ? -x : x;
  return;
}
int t[200051], flag, n;
int main() {
  read(n);
  for (int i = 1; i <= n; i++) read(t[i]);
  for (int i = 1; i <= n; i++) {
    if (t[i] > t[i - 1] && flag == 0) continue;
    if (t[i] < t[i - 1] && i != 1) flag = 1;
    if (flag == 1 && t[i] > t[i - 1]) {
      printf("NO");
      return 0;
    }
  }
  printf("YES");
  return 0;
}
