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

inline void solve() {
    int n;
    cin >> n;

    vector<int> a(n);
    for (int &i : a)
        cin >> i;
    
    for (int i = 0; i < n; ++i)
        if (a[i] < 0 || a[i] > 2) {
            cout << "NO\n";
            return;
        }

    bool ok = 1;
    for (int i = 0; i < n - 1; ++i) {
        if (a[i] == 0) 
           continue;
        else if (a[i] == 1) {
            if (a[i + 1] == 0)
                a[i + 1] = 1;
            else
                a[i + 1] -= 1;

        }
        else {
            ok = 0;
            break;
        }
    }
    cout << (ok && a.back() == 0 ? "YES" : "NO") << '\n';
    return;
}

signed main() {
    cin.tie(0)->sync_with_stdio(0);
    int t = 1;

    while (t--)
        solve();
    return 0;
}