#include <iostream>
#include <algorithm>
#include <vector>
#include <set>
using namespace std;
template<class T> inline void YES(T condition){ if(condition) cout << "YES" << endl; else cout << "NO" << endl; }
template<class T> inline void Yes(T condition){ if(condition) cout << "Yes" << endl; else cout << "No" << endl; }
template<class T> inline void POSS(T condition){ if(condition) cout << "POSSIBLE" << endl; else cout << "IMPOSSIBLE" << endl; }
template<class T> inline void Poss(T condition){ if(condition) cout << "Possible" << endl; else cout << "Impossible" << endl; }
template<class T> inline void First(T condition){ if(condition) cout << "First" << endl; else cout << "Second" << endl; }
template<class T = string, class U = char>int character_count(T text, U character){ int ans = 0; for(U i: text){ ans += (i == character); } return ans; }
long power(long base, long exponent, long module){ if(exponent % 2){ return power(base, exponent - 1, module) * base % module; }else if(exponent){ long root_ans = power(base, exponent / 2, module); return root_ans * root_ans % module; }else{ return 1; }}
struct position{ int y, x; }; position move_pattern[4] = {{-1, 0}, {0, 1}, {1, 0}, {0, -1}}; // double euclidean(position first, position second){ return sqrt((second.x - first.x) * (second.x - first.x) + (second.y - first.y) * (second.y - first.y)); }
template<class T, class U> string to_string(pair<T, U> x){ return to_string(x.first) + "," + to_string(x.second); }
template<class itr> void array_output(itr start, itr goal){ string ans; for(auto i = start; i != goal; i++){ ans += to_string(*i) + " "; } ans.pop_back(); cout << ans << endl; }
template<class itr> void cins(itr start, itr goal){ for(auto i = start; i != goal; i++){ cin >> (*i); } }
template<class T> T gcd(T a, T b){ if(a && b){ return gcd(min(a, b), max(a, b) % min(a, b)); }else{ return a; }} template<class T> T lcm(T a, T b){ return a / gcd(a, b) * b; }
#define mod long(1e9 + 7)
#define all(x) (x).begin(), (x).end()
#define bitcount(n) __builtin_popcountl(long(n))
#define fcout cout << fixed << setprecision(10)
#define highest(x) (63 - __builtin_clzl(x))
struct combination{ vector<long> fact, inv; combination(int sz) : fact(sz + 1), inv(sz + 1){ fact[0] = 1; for(int i = 1; i <= sz; i++){ fact[i] = fact[i - 1] * i % mod; } inv[sz] = power(fact[sz], mod - 2, mod); for(int i = sz - 1; i >= 0; i--){ inv[i] = inv[i + 1] * (i + 1) % mod; } } long C(int p, int q) const{ if(q < 0 || p < q) return 0; return (fact[p] * inv[q] % mod * inv[p - q] % mod); } };

int main(){
    int N;
    cin >> N;
    int A[N];
    cins(A, A + N);
    for(int i = 0; i < N - 1; i++){
        if(A[i] >= A[i + 1]){
            goto next;
        }
    }
    cout << 1 << endl;
    return 0;
next:
    for(int i = 0; i < N; i++){
        A[i] = min(A[i], int(2e5 + 10) + i);
    }
    int left = 0, right = int(1e6); // (left, right]
    while(left + 1 < right){
        int middle = (left + right) / 2;
        int now[int(5e5 + 100)];
        for(int i = 0; i < int(5e5 + 100); i++){
            now[i] = 0;
        }
        set<int, greater<int>> updated;
        for(int i = 1; i < N; i++){
            if(A[i - 1] >= A[i]){
                now[A[i] - 1]++;
                updated.insert(A[i] - 1);
                for(int j = A[i] - 1; now[j] == middle; j--){
                    now[j] = 0;
                    if(j == 0){
                        goto no;
                    }
                    now[j - 1]++;
                    updated.insert(j - 1);
                }
            }
            while(!updated.empty() && (*(updated.begin())) > A[i] - 1){
                now[(*(updated.begin()))] = 0;
                updated.erase(updated.begin());
            }
        }
        right = middle;
        continue;
    no:
        left = middle;
    }
    cout << right << endl;
}



Measured memory_bytes: 11776