#include <bits/stdc++.h> using namespace std; #define N 1005

int main() { int n, a[N], st[N] = {}; cin >> n; for (int i = 1; i <= n; ++i) cin >> a[i]; for (int i = 1; i <= n; ++i) { for (int j = a[i] + 1; j <= n; ++j) { if (st[j] == 1) { cout << "NO"; return 0; } } for (int j = 1; j < a[i]; ++j) { if (st[j] == 0) st[j] = 1; } st[a[i]] = 2; } cout << "YES"; return 0; }