#include <bits/stdc++.h>
using namespace std;
#define rep(i,l,r) for(int i=(int)l;i<(int)r;i++)

int main(){
  //subtask2
  int n;cin>>n;
  vector<int> h(n);
  rep(i,0,n)cin>>h[i];
  int cnt1=0,cnt2=0;
  rep(i,1,n-1){
    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;
}