#include <bits/stdc++.h>
#define ll long long
#define INF 1000000005
#define MOD 1000000007
#define EPS 1e-10
#define rep(i,n) for(int i=0;i<(int)(n);++i)
#define rrep(i,n) for(int i=(int)(n)-1;i>=0;--i)
#define srep(i,s,t) for(int i=(int)(s);i<(int)(t);++i)
#define each(a,b) for(auto& (a): (b))
#define all(v) (v).begin(),(v).end()
#define len(v) (int)(v).size()
#define zip(v) sort(all(v)),v.erase(unique(all(v)),v.end())
#define cmx(x,y) x=max(x,y)
#define cmn(x,y) x=min(x,y)
#define fi first
#define se second
#define pb push_back
#define show(x) cout<<#x<<" = "<<(x)<<endl
#define sar(a,n) {cout<<#a<<":";rep(pachico,n)cout<<" "<<a[pachico];cout<<endl;}

using namespace std;

template<typename S,typename T>auto&operator<<(ostream&o,pair<S,T>p){return o<<"{"<<p.fi<<","<<p.se<<"}";}
template<typename T>auto&operator<<(ostream&o,set<T>s){for(auto&e:s)o<<e<<" ";return o;}
template<typename S,typename T,typename U>
auto&operator<<(ostream&o,priority_queue<S,T,U>q){while(!q.empty())o<<q.top()<<" ",q.pop();return o;}
template<typename K,typename T>auto&operator<<(ostream&o,map<K,T>&m){for(auto&e:m)o<<e<<" ";return o;}
template<typename T>auto&operator<<(ostream&o,vector<T>v){for(auto&e:v)o<<e<<" ";return o;}
void ashow(){cout<<endl;}template<typename T,typename...A>void ashow(T t,A...a){cout<<t<<" ";ashow(a...);}
template<typename S,typename T,typename U>
struct TRI{S fi;T se;U th;TRI(){}TRI(S f,T s,U t):fi(f),se(s),th(t){}
bool operator<(const TRI&_)const{return(fi==_.fi)?((se==_.se)?(th<_.th):(se<_.se)):(fi<_.fi);}};
template<typename S,typename T,typename U>
auto&operator<<(ostream&o,TRI<S,T,U>&t){return o<<"{"<<t.fi<<","<<t.se<<","<<t.th<<"}";}

typedef pair<int, int> P;
typedef pair<ll, ll> pll;
typedef TRI<int, int, int> tri;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<P> vp;
typedef vector<double> vd;
typedef vector<string> vs;

const int MAX_N = 200005;

int n;
ll a[MAX_N];

bool ok(int cri){
    // show(cri);
    vector<pll> vec;
    ll prv = 0;
    rep(i,n){
        if(prv < a[i]){
            vec.emplace_back(a[i]-prv, 0);
        }else{
            bool check = false;
            ll cur = 0;
            while(!vec.empty()){
                pll& p = vec.back();
                if(prv - p.fi >= a[i]){
                    prv -= p.fi;
                    vec.pop_back();
                }else if(check){
                    if(p.se < cri - 1){
                        if(p.fi > 1){
                            p.fi = p.fi - 1;
                            vec.emplace_back(1, p.se + 1);
                        }else{
                            assert(p.fi == 1);
                            p.se = p.se + 1;
                        }
                        if(cur > 0) vec.emplace_back(cur, 0);
                        check = false;
                        break;
                    }else{
                        cur += p.fi;
                        vec.pop_back();
                    }
                }else if(p.se < cri - 1){
                    if(prv - p.fi + 1 == a[i]){
                        p.fi = 1, p.se = p.se + 1;
                    }else{
                        p.fi = a[i] - (prv - p.fi) - 1;
                        vec.emplace_back(1, p.se + 1);
                    }
                    break;
                }else{
                    cur = a[i] - (prv - p.fi);
                    check = true;
                    vec.pop_back();
                }
            }
            if(check) return false;
        }
        // show(vec);
        prv = a[i];
    }
    return true;
}

int main()
{
    cin.tie(0);
    ios::sync_with_stdio(false);
    cin >> n;
    rep(i,n){
        cin >> a[i];
    }
    int l = 0, r = n;
    while(r-l>1){
        int mid = (l+r)/2;
        if(ok(mid)){
            r = mid;
        }else{
            l = mid;
        }
    }
    cout << r << "\n";
    return 0;
}


Measured memory_bytes: 9104