//header
#ifdef LOCAL
    #include "cxx-prettyprint-master/prettyprint.hpp"
    #define print(x) cout << x << endl
#else
    #define print(...) 42
#endif
    #pragma GCC optimize("Ofast")
    #include <bits/stdc++.h>
    //types
    using namespace std;
    using ll = long long;
    typedef pair < ll , ll >P;
    typedef vector<ll> vl;
    template< typename T >
    using mat = vector< vector< T > >;
    template <std::uint_fast64_t Modulus> class modint {
        using u64 = std::uint_fast64_t;

        public:
        u64 a;

        constexpr modint(const u64 x = 0) noexcept : a(x % Modulus) {}
        constexpr u64 &value() noexcept { return a; }
        constexpr const u64 &value() const noexcept { return a; }
        constexpr modint operator+(const modint rhs) const noexcept {
            return modint(*this) += rhs;
        }
        constexpr modint operator-(const modint rhs) const noexcept {
            return modint(*this) -= rhs;
        }
        constexpr modint operator*(const modint rhs) const noexcept {
            return modint(*this) *= rhs;
        }
        constexpr modint operator/(const modint rhs) const noexcept {
            return modint(*this) /= rhs;
        }
        constexpr modint &operator+=(const modint rhs) noexcept {
            a += rhs.a;
            if (a >= Modulus) {
            a -= Modulus;
            }
            return *this;
        }
        constexpr modint &operator-=(const modint rhs) noexcept {
            if (a < rhs.a) {
            a += Modulus;
            }
            a -= rhs.a;
            return *this;
        }
        constexpr modint &operator*=(const modint rhs) noexcept {
            a = a * rhs.a % Modulus;
            return *this;
        }
        constexpr modint &operator/=(modint rhs) noexcept {
            u64 exp = Modulus - 2;
            while (exp) {
            if (exp % 2) {
                *this *= rhs;
            }
            rhs *= rhs;
            exp /= 2;
            }
            return *this;
        }
        };
    //abbreviations
    #define all(x) (x).begin(), (x).end()
    #define reverse(x) (x).rbegin(), (x).rend()
    #define rep2(i,a,b) for(ll i=(a);i<(b);++i)
    #define rep(i,n) for(ll i=0;i<(n);++i)
    #define SZ(x) ((ll)(x).size())
    #define pb(x) push_back(x)
    #define ff first
    #define ss second
    #define mp make_pair
    //functions
    ll gcd(ll a, ll b) { return b ? gcd(b, a%b) : a; }
    template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
    template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
    template< typename T >
    T mypow(T x, ll n) {
        T ret = 1;
        while(n > 0) {
            if(n & 1) (ret *= x);
            (x *= x);
            n >>= 1;
        }
        return ret;
    }
    uint64_t my_rand(void) {
        static uint64_t x = 88172645463325252ULL;
        x = x ^ (x << 13); x = x ^ (x >> 7);
        return x = x ^ (x << 17);
    }
    //graph template
    template< typename T >
    struct edge {
        int src, to;
        T cost;

        edge(int to, T cost) : src(-1), to(to), cost(cost) {}

        edge(int src, int to, T cost) : src(src), to(to), cost(cost) {}

        edge &operator=(const int &x) {
        to = x;
        return *this;
    }

    operator int() const { return to; }
    };

    template< typename T >
    using Edges = vector< edge< T > >;
    template< typename T >
    using WeightedGraph = vector< Edges< T > >;
    using UnWeightedGraph = vector< vector< int > >;

//constant
#define INF 1'010'000'000'000'000'017LL
#define mod 1000003LL
typedef modint<mod> mint;
#define eps 0.000001
//library
bool C(vl &A, ll mid){
    int N = SZ(A);
    vector<P> now = {mp(-1, 0)};
    rep(i, N){
        ll a = A[i];
        if((*(now.end()-1)).second<a){
            now.pb(mp(0, a));
            continue;
        }
        P end;
        while((*(now.end()-1)).second>=a){
            end = *(now.end()-1);
            now.pop_back();
        }
        end.second = a-1;
        if(end.second>(*(now.end()-1)).second){
            now.pb(end);
        }
        if(end.first == mid-1){
            auto iter = (now.end()-1);
            while((*iter).first == mid-1){
                (*iter).first = 0;
                iter--;
            }
            if((*iter).second == 0){
                return false;
            }
            if((*iter).second == (*(iter-1)).second+1){
                (*iter).first++;
            }else{
            now.insert(iter+1, mp(((*iter).first+1), (*iter).second));
            (*iter).second--;
            }
            now.pb(mp(0, a));
        }
        else{
            now.pb(mp(end.first+1, a));
        }       
    }
    return true;
}
//main
int main(){
    int N; cin>>N;
    vl A(N);
    rep(i, N)cin>>A[i];
    ll ng = 0, ok = INF;
    while(ok-ng>1){
        ll mid = (ok+ng)/2;
        if(C(A, mid))ok = mid;
        else ng = mid;
    }
    cout << ok <<endl;
}

Measured memory_bytes: 9088