#define NguyenDangQuan the_author

#include <bits/stdc++.h>
#define all(x) x.begin(),x.end()
#define mset(x, i) memset(x,i,sizeof(x))
#define elif else if
#define heap priority_queue
#define fi first
#define se second
#define pb push_back
#define ld long double
#define ll long long
#define ull unsigned long long
using namespace std;

int typetest;

inline void fastIOfileinput(){
    ios_base:: sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    typetest = 0;
}

template<class T>
void read(T &x){
    register int c;
    x = (T)0;
    while ((c = getchar()) <= 47 || c >= 58);
    for (; (c > 47 && c < 58); c = getchar()){
        x = (x << 3) + (x << 1) + (T)(c - 48);
    }
}

const int N = 1e5 + 2;
const int Inf = 1e7 + 1;
int n, a[N];
int stmax[N * 4];

void Enter(){
    //cin >> n;
    read(n);
    for(int i = 1; i <= n; ++i)
        //cin >> a[i];
        read(a[i]);
}

void build(int id, int l, int r){
    if(l == r){
        stmax[id] = a[l];
        return;
    }
    build(id << 1, l, (l + r) / 2);
    build(id << 1 | 1, (l + r) / 2 + 1, r);
    stmax[id] = max(stmax[id << 1], stmax[id << 1 | 1]);
}

int get(int id, int l, int r, int a, int b){
    if(l > b || r < a)
        return -Inf;
    if(l >= a && r <= b)
        return stmax[id];
    int s = get(id << 1, l, (l + r) / 2, a, b),
        p = get(id << 1 | 1, (l + r) / 2 + 1, r, a, b);
    return max(s, p);
}

int sum[4 * N];

void updat(int id, int l, int r, int pos, int val){
    if(l > pos || r < pos)
        return;
    if(l == pos && r == pos){
        sum[id] = val;
        return;
    }
    updat(id << 1, l, (l + r) / 2, pos, val);
    updat(id << 1 | 1, (l + r) / 2 + 1, r, pos, val);
    sum[id] = sum[id << 1] + sum[id << 1 | 1];
}

int que(int id, int l, int r, int a, int b){
    if(l > b || r < a)
        return 0;
    if(l >= a && r <= b)
        return sum[id];
    return que(id << 1, l, (l + r) / 2, a, b) + que(id << 1 | 1, (l + r) / 2 + 1, r, a, b);
}

void solve(){
    build(1, 1, n);
    int res = 0;
    int last[N];
    for(int i = 1; i <= n; ++i){
        updat(1, 1, n, i, 1);
        updat(1, 1, n, last[a[i]], 0);
        last[a[i]] = i;
        for(int j = max(max(a[i], res + 1), i - last[1] + 1); j <= i && que(1, 1, n, i - j + 1, i) == j;){
            int s = get(1, 1, n, i - j + 1, i);
            if(s == j)
                res = max(res, j),
                ++j;
            else
                j = s;
        }
    }
    cout << res;
}

signed main(){
    fastIOfileinput();
    if(typetest){
        int t;
        cin >> t;
        while(t--){
            Enter();
            solve();
        }
    }
    else{
        Enter();
        solve();
    }
}