//code by p1ckle/sft
#include <bits/stdc++.h>
#define int long long int
#define pii pair<int, int>
#define F first
#define S second
#define mid ((start+end)/2)
#define all(x) x.begin(), x.end()
#define ub(a, b) upper_bound(all(a), b)
#define lb(a, b) lower_bound(all(a), b)
#define pb(a) push_back(a)

using namespace std;

const int inf = 1e18, mod = 1e9+7;
int max (vector<int> a) {int t = -inf; for (int i : a) t = max(t, i); return t;}

int n, m, q, a[200005], b, c;

signed main() {

    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    cin >> n;
    for (int i = 1; i <= n; i++) cin >> a[i];
    for (int i = 1; i < n; i++) {

        if (abs(a[i]-a[i+1]) != 1) {cout << "No"; return 0;}

    }
    cout << "Yes";

}