#include <bits/stdc++.h>
using namespace std;

int main(){
  //subtask3
  int n;cin>>n;
  vector<int> h(n);
  for(int i=0;i<n;i++)cin>>h[i];
  int cnt1=0,cnt2=0;
  for(int i=1;i<n-1;i++){
    if(h[i-1]>h[i]&&h[i]<h[i+1])cnt1++;
    else if(h[i-1]<h[i]&&h[i]>h[i+1])cnt2++;
  }
  cout<<(cnt1==2&&cnt2==1?"Yes":"No")<<endl;
}