#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];
  rep(x,1,n-1)rep(y,x+1,n-1)rep(z,y+1,n-1){
    bool ok=1;
    rep(i,0,x)if(h[i]<h[i+1])ok=0;
    rep(i,x,y)if(h[i]>h[i+1])ok=0;
    rep(i,y,z)if(h[i]<h[i+1])ok=0;
    rep(i,z,n-1)if(h[i]>h[i+1])ok=0;
    if(ok){
      cout<<"Yes"<<endl;
      return 0;
    }
  }
  cout<<"No"<<endl;
}