#include <bits/stdc++.h>
using namespace std;


#define li          long long int
#define rep(i,to)   for(li i=0;i<((li)(to));i++)
#define repp(i,start,to)    for(li i=(li)(start);i<((li)(to));i++)
#define pb          push_back
#define sz(v)       ((li)(v).size())
#define bgn(v)      ((v).begin())
#define eend(v)     ((v).end())
#define allof(v)    (v).begin(), (v).end()
#define dodp(v,n)       memset(v,(li)n,sizeof(v))
#define bit(n)      (1ll<<(li)(n))
#define mp(a,b)     make_pair(a,b)
#define rin rep(i,n)
#define EPS 1e-12
#define ETOL 1e-8
#define MOD 1000000007
typedef pair<li, li> PI;

#define INF bit(60)

#define DBGP 1


#define idp if(DBGP)
#define F first
#define S second
#define p2(a,b)     idp cout<<a<<"\t"<<b<<endl
#define p3(a,b,c)       idp cout<<a<<"\t"<<b<<"\t"<<c<<endl
#define p4(a,b,c,d)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<endl
#define p5(a,b,c,d,e)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<endl
#define p6(a,b,c,d,e,f)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<endl
#define p7(a,b,c,d,e,f,g)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<endl
#define p8(a,b,c,d,e,f,g,h)     idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<endl
#define p9(a,b,c,d,e,f,g,h,i)       idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<endl
#define p10(a,b,c,d,e,f,g,h,i,j)        idp cout<<a<<"\t"<<b<<"\t"<<c<<"\t"<<d<<"\t"<<e<<"\t"<<f<<"\t"<<g<<"\t"<<h<<"\t"<<i<<"\t"<<j<<endl
#define foreach(it,v)   for(__typeof((v).begin()) it=(v).begin(); it!=(v).end(); ++it)
#define p2p(x)      idp p2((x).F, (x).S)
#define dump(x,n)   idp{rep(i,n){cout<<x[i]<<" ";}puts("");}
#define dump2(x,n)  idp{rep(i,n){cout<<"["<<x[i].F<<" , "<<x[i].S<<"] ";}puts("");}
#define dumpi(x)    idp{foreach(it, x){cout<<(*it)<<" ";}puts("");}
#define dumpi2(x)   idp{foreach(it, x){cout<<"["<<(it)->F<<" , "<<(it)->S<<"] ";}puts("");}

#define read2d(a,w,h)   rep(i,h)rep(j,w)cin>>a[i][j]
#define dump2d(a,w,h)   rep(i,h){rep(j,w)cout<<a[i][j]<<" ";puts("");}

typedef pair<li, li> PI;


li res;
li a[200100];
li n;
li state[200100];

// lazy segment tree

li segtree_val[550000];
li segtree_update_time[550000];
li leaf_size = 0;

inline void init_segtree(li num) {
    leaf_size = 1;
    while (leaf_size < num)leaf_size *= 2;
    rep(i, leaf_size * 2 - 1) {
        segtree_val[i] = 0;
        segtree_update_time[i] = -1;
    }
}

// update [a, b), now in [l, r)
inline void range_update(li a, li b, li k, li l, li r, li val, li timing) {
    if (r <= a || b <= l)return;
    if (a <= l && r <= b) {
        segtree_val[k] = val;
        segtree_update_time[k] = timing;
    } else {
        range_update(a, b, k * 2 + 1, l, (l + r) / 2, val, timing);
        range_update(a, b, k * 2 + 2, (l + r) / 2, r, val, timing);
    }
}

inline void range_update_(li a, li b, li val, li timing) {
    range_update(a, b, 0, 0, leaf_size, val, timing);
}

inline li query(li i) {
    i += leaf_size - 1;
    li res = segtree_val[i];
    li res_time = segtree_update_time[i];
    while (i > 0) {
        i = (i - 1) / 2;
        if (res_time < segtree_update_time[i]) {
            res = segtree_val[i];
            res_time = segtree_update_time[i];
        }
    }
    return res;
}

inline li inc(li x, li num, li timing) {
    if (x < 0)return INF;
    if (num == 1)return INF;
    li now = query(x);
    if (now < num) {
        range_update_(x, x + 1, now + 1, timing);
        return now + 1;
    }
    range_update_(x, x + 1, 1, timing);
    return inc(x - 1, num, timing);
}

inline bool ok(li num) {
    if (num == 0)return false;
    init_segtree(200020);
    li res = 1;

    li prev_len = 0;
    rin{
        if (prev_len < a[i]) {
            range_update_(prev_len, a[i], 1, i);
            //repp(j, prev_len, a[i]) {
            //    state[j] = 1;
            //}
        } else if (prev_len == a[i]) {
            res = max(res, inc(a[i] - 1, num, i));
        } else {
            // prev_len > a[i]
            res = max(res, inc(a[i] - 1, num, i));
        }
        prev_len = a[i];
        if (res > num)return false;
    }
    return true;
}

int main() {
    /*
    init_segtree(8);
    range_update_(1, 5, 3, 0);
    dump(segtree_val, 15);
    dump(segtree_update_time, 15);
    range_update_(3, 8, 2, 1);
    puts("---");
    dump(segtree_val, 15);
    dump(segtree_update_time, 15);
    rep(i, 8)cout << query(i) << endl;


    return 0;
    */
    cin >> n;
    rin{
        cin >> a[i];
        a[i] = min(a[i], 200000ll);
    }

    li left = 0, right = n;
    while (left + 1 < right) {
        li mid = (left + right) / 2;
        if (ok(mid)) {
            right = mid;
        } else {
            left = mid;
        }
    }
    cout << right << endl;

    return 0;
}

Measured memory_bytes: 11904