#include <iostream>

using namespace std;

int main(){
  int count=0;
  int n;
  cin >> n;
  int a[n];
  bool flag = false;

  for (int i = 0; i < n ; ++i)
    cin >> a[i];

  for (int i = 1; i < n; ++i){
      if ((a[i] >= 0 && a[i-1] >= 0) || (a[i] < 0 && a[i-1] < 0))
        flag = true;
    }

  if (flag == true){
      cout << "YES";
    } else {
      cout << "NO";
    }

  return 0;
}