#include <bits/stdc++.h>

using namespace std;

vector<string> split_string(string);

/*
 * Complete the canModify function below.
 */
string canModify(vector<int> a) {
    int n = a.size();
    for (int i = 0; i < n; i++) {
        bool ok = true;
        for (int j = 1; j < i; j++)
            if (a[j] < a[j-1]) {
                ok = false; break;
            }
        for (int j = i+1; j+1 < n; j++)
            if (a[j] > a[j+1]) {
                ok = false; break;
            }
        if (i > 0 && i+1 < n) ok = ok && (a[i-1] <= a[i+1]);
        if (ok) return "YES";
    }
    return "NO";
}

int main()
{
    ofstream fout(getenv("OUTPUT_PATH"));

    int n;
    cin >> n;
    cin.ignore(numeric_limits<streamsize>::max(), '\n');

    string a_temp_temp;
    getline(cin, a_temp_temp);

    vector<string> a_temp = split_string(a_temp_temp);

    vector<int> a(n);

    for (int a_itr = 0; a_itr < n; a_itr++) {
        int a_item = stoi(a_temp[a_itr]);

        a[a_itr] = a_item;
    }

    string result = canModify(a);

    fout << result << "\n";

    fout.close();

    return 0;
}

vector<string> split_string(string input_string) {
    string::iterator new_end = unique(input_string.begin(), input_string.end(), [] (const char &x, const char &y) {
        return x == y and x == ' ';
    });

    input_string.erase(new_end, input_string.end());

    while (input_string[input_string.length() - 1] == ' ') {
        input_string.pop_back();
    }

    vector<string> splits;
    char delimiter = ' ';

    size_t i = 0;
    size_t pos = input_string.find(delimiter);

    while (pos != string::npos) {
        splits.push_back(input_string.substr(i, pos - i));

        i = pos + 1;
        pos = input_string.find(delimiter, i);
    }

    splits.push_back(input_string.substr(i, min(pos, input_string.length()) - i + 1));

    return splits;
}