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

#ifdef DEBUG
#define fast
#else
#define fast cin.tie(0)->sync_with_stdio(0)
#define endl '\n'
#define cerr if(1);else cerr
#endif
#define _ <<' '<<
#define ALL(v) v.begin(),v.end()
#define ft first
#define sd second

using ll = long long;
using ld = long double;
using pii = pair<int,int>;

signed main() {
    fast;

    int n;
    cin >> n;
    vector<int> v(n);
    for (auto &x: v) cin >> x;
    bool can = true;

    for (int i = 1; i < n; i++) {
        if (v[i-1] > 0)
            if (v[i] == 0)
                v[i] = 1;
            else
                v[i]--;
        if (v[i] == 2) can = false;
    }
    if (v[0] == 2 or v[n-1] != 0) can = false;

    cout << (can ? "YES" : "NO") << endl;

    return 0;
}