#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
#define SZ(x) ((int)((x).size()))

template <typename T1, typename T2>
string print_iterable(T1 begin_iter, T2 end_iter, int counter) {
    bool done_something = false;
    stringstream res;
    res << "[";
    for (; begin_iter != end_iter and counter; ++begin_iter) {
        done_something = true;
        counter--;
        res << *begin_iter << ", ";
    }
    string str = res.str();
    if (done_something) {
        str.pop_back();
        str.pop_back();
    }
    str += "]";
    return str;
}

vector<int> SortIndex(int size, std::function<bool(int, int)> compare) {
    vector<int> ord(size);
    for (int i = 0; i < size; i++) ord[i] = i;
    sort(ord.begin(), ord.end(), compare);
    return ord;
}

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

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

template <typename S, typename T>
ostream& operator <<(ostream& out, const pair<S, T>& p) {
    out << "{" << p.first << ", " << p.second << "}";
    return out;
}

template <typename T>
ostream& operator <<(ostream& out, const vector<T>& v) {
    out << "[";
    for (int i = 0; i < (int)v.size(); i++) {
        out << v[i];
        if (i != (int)v.size()-1) out << ", ";
    }
    out << "]";
    return out;
}

template<class TH>
void _dbg(const char* name, TH val){
    clog << name << ": " << val << endl;
}
template<class TH, class... TA>
void _dbg(const char* names, TH curr_val, TA... vals) {
    while(*names != ',') clog << *names++;
    clog << ": " << curr_val << ", ";
    _dbg(names+1, vals...);
}

#if DEBUG && !ONLINE_JUDGE
    ifstream input_from_file("input.txt");
    #define cin input_from_file

    #define dbg(...) _dbg(#__VA_ARGS__, __VA_ARGS__)
    #define dbg_arr(x, len) clog << #x << ": " << print_iterable(x, x+len, -1) << endl;
#else
    #define dbg(...)
    #define dbg_arr(x, len)
#endif

///////////////////////////////////////////////////////////////////////////
//////////////////// DO NOT TOUCH BEFORE THIS LINE ////////////////////////
///////////////////////////////////////////////////////////////////////////

int ALPHA;

struct Chunk {
    int start;
    int end;
    int val;
    Chunk(int start, int end, int val): start(start), end(end), val(val) {}
    int size() { return end-start+1; }
};

struct CompressedString{
    vector<Chunk> vec;
    void increase(int new_len) {
        if (vec.back().val == 0) vec.back().end = new_len-1;
        else vec.emplace_back(vec.back().end+1, new_len - 1, 0);
    }
    void decrease(int new_len) {
        while (vec.back().start >= new_len) vec.pop_back();
        vec.back().end = new_len-1;
    }
    bool add1() {
        int initial_len = vec.back().end + 1;
        while (!vec.empty() and vec.back().val == ALPHA-1) vec.pop_back();
        if (vec.empty()) return false;
        
        int x = vec.back().val;
        int e = vec.back().end;
        if (vec.back().size() == 1) vec.pop_back();
        else vec.back().end--;
        vec.emplace_back(e, e, x+1);
        if (e+1 < initial_len) increase(initial_len);

        return true;
    }
    bool change_size(int new_len) {
        dbg(vec.back().end, new_len);
        if (vec.back().end == new_len - 1) {
            dbg("SAME");
            return add1();
        } else if (vec.back().end > new_len-1) {
            decrease(new_len);
            dbg("DECREASE");
            return add1();
        }
        else if (vec.back().end < new_len-1) {
            dbg("INCREASE");
            increase(new_len);
            return true;
        }
        return false;
    }
};

int N;
vector<int> A;

bool works() {
    CompressedString CS;
    CS.vec.emplace_back(0, A[0]-1, 0);
    for (int i = 1; i < N; i++) {
        if (!CS.change_size(A[i])) return false;
    }
    return true;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0); // Remove in problems with online queries!

    cin >> N;
    A.resize(N);
    for (int i = 0; i < N; i++) cin >> A[i];

    
    // ALPHA = 2;
    // cout << works() << endl;
    // return 0;

    int l = 1;
    int r = 2e5+10;
    while (l < r) {
        ALPHA = (l+r)/2;
        if (works()) r = ALPHA;
        else l = ALPHA+1;
    }
    cout << l << "\n";
}


Measured memory_bytes: 1024