// #include <bits/stdc++.h>
#include <iostream> // cin, cout, string
#include <algorithm> // sort, min/max, upper/lower_bound, transform, count/copy_if
#include <vector>
#include <utility> // pair, swap
#include <functional>
#include <iomanip> // setprecision
#include <map>
#include <set> // multiset
#include <unordered_map>
#include <unordered_set>
#include <tuple>
#include <stack>
#include <queue> // priority_queue
#include <deque>
#include <bitset>
#include <array>
#include <list>
#include <forward_list>
#include <complex>
#include <ratio>
#include <cassert>
#include <numeric> // accumulate, itoa
#include <random> // mt19937
#include <iterator>

using namespace std;

// #define int long long

using i64 = int64_t;
using ll = long long;
using ld = long double;

template<typename T> using vec = vector<T>;
template<typename T> using mat = vec<vec<T>>;
template<typename T> using dic = unordered_map<string, T>;
template<typename T> using pqg = priority_queue<T, vector<T>, greater<T>>; // dijkstra
// template<typename T, typename U> using P = pair<T, U>;
// template<typename... Args> using func = function<Args...>;
// template<typename T> using pq = priority_queue<T>;

using pii = pair<int, int>;
using ppi = pair<pii, int>;
using pip = pair<int, pii>;
using ppp = pair<pii, pii>;

using vi = vec<int>;
using mi = mat<int>; // Graph
using vp = vec<pii>;

constexpr int IINF =  0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
// constexpr int INF = sizeof(int) == sizeof(long long) ? LINF : IINF;
constexpr long long MOD = 1e9+7;

#define _overload(_1,_2,_3,name,...) name
#define _rep1(i,a,b) for(int i=(a), _##i=(b);i<_##i;++i)
#define _rep2(i,n) _rep1(i,0,n)
#define _rep3(n) _rep2(i,n)
#define rep(...) _overload(__VA_ARGS__,_rep1,_rep2,_rep3)(__VA_ARGS__)
#define _rrep1(i,a,b) for(int i=(b)-1, _##i=(a);i>=_##i;--i)
#define _rrep2(i,n) _rrep1(i,0,n)
#define _rrep3(n) _rrep2(i,n)
#define rrep(...) _overload(__VA_ARGS__,_rrep1,_rrep2,_rrep3)(__VA_ARGS__)

#define each(i,a) for(auto&& i:a)
#define each2(i,a) for(const auto& i:a)

#define pb push_back
// emplace_back(*) == push_back(constructor(*))
#define eb emplace_back
#define pob pop_back
#define pf push_front
#define pof pop_front

#define np next_permutation
#define pp prev_permutation

// make_pair(A, B) == pair<T, U>(A, B)
#define mp make_pair
// mt(...) == tie(...)
#define mt make_tuple

#define ft first
#define sd second

#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define sz(a) (int)(a).size()
#define rev(a) reverse(all(a))
#define rs(a,l) (a).resize(l)

#define upper(s) transform(all(s), (s).begin(), toupper)
#define lower(s) transform(all(s), (s).begin(), tolower)

#define sum(a) accumulate(all(a), 0)
#define elif else if
#define unless(a) if(!(a))
#define qt(a) #a

#define endl '\n'
#define dec(n) fixed << setprecision(n)
#define zfl(n)  setfill(‘0’) << setw(n)

template<typename T=int, T l=0, T r=(T)MOD>
auto rnd = bind(uniform_int_distribution<T>(l, r), mt19937(chrono::steady_clock::now().time_since_epoch().count()));

template<typename T> inline constexpr T gcd2(T a, T b) { if(a==0 || b==0) return 0; while(b) { a = a%b; swap(a, b); } return a; }
template<typename T> inline constexpr T extgcd2(T a, T b, T &x, T &y) { x = 1; y = 0; while (b) { T t = a / b; a -= t * b; swap(a, b); x -= t * y; swap(x, y); } return a; }
template<typename T> inline constexpr T lcm(T a, T b) { return a / gcd(a, b) * b; }

template<typename T> inline constexpr bool chmax(T &a, const T &b) { if (a < b) { a = b; return true; } return false; }
template<typename T> inline constexpr bool chmin(T &a, const T &b) { if (b < a) { a = b; return true; } return false; }

template<typename A, typename B> istream &operator>>(istream &is, pair<A, B> &p) { is >> p.first; is >> p.second; return is; }
template<typename A, typename B> ostream &operator<<(ostream &os, const pair<A, B> &p) { os << p.first << " " << p.second; return os; }

template<typename T> istream &operator>>(istream &is, vector<T> &v) { for(auto&& x : v) is >> x; return is; }
template<typename T> ostream &operator<<(ostream &os, const vector<T> &v) { for(int i=0; i<(int)v.size(); ++i) cout << v[i] << ( i+1 != (int)v.size() ? " " : "" ); return os; }

#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << endl
#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg> void __f(const char* name, const Arg& arg) { cerr << name << " : " << arg << endl; }
template <typename Arg, typename... Args> void __f(const char* names, const Arg& arg, const Args&... args)
{ const char* comma = strchr(names + 1, ','); cerr.write(names, comma - names) << " : " << arg << " | "; __f(comma+1, args...); }

inline auto yes(const bool &c) { return c ? "yes" : "no"; }
inline auto Yes(const bool &c) { return c ? "Yes" : "No"; }
inline auto YES(const bool &c) { return c ? "YES" : "NO"; }
inline auto Possible(const bool &c) { return c ? "Possible" : "Impossible"; }
inline auto POSSIBLE(const bool &c) { return c ? "POSSIBLE" : "IMPOSSIBLE"; }

constexpr int lim = 100007; // 10^5+7

int N;
i64 H[lim];

bool check() {
  i64 min_h = H[N-1];

  rrep(i, N-1) {
    chmin(min_h, H[i+1]);
    if(H[i] > min_h) {
      H[i] -= 1;
      if(H[i] > min_h) return false;
    }
  }

  return true;
}

signed main() {
  cin.tie(0); cout.tie(0);
  ios_base::sync_with_stdio(false);

  cin >> N;
  rep(N) cin >> H[i];

  cout << Yes(check()) << endl;

  return 0;
}


Measured memory_bytes: 1024