#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
bool check(int k, int n, const vector<int>& A) {
// ties[len] = how many models have a tie of length 'len'
vector<int> ties(22, 0);
ties[1] = k;
bool prev_ignored = false;
for (int i = 0; i < n; ++i) {
int x = A[i];
int best_len = -1;
// Find the largest tie length <= x
for (int l = x; l >= 1; --l) {
if (ties[l] > 0) {
best_len = l;
break;
}
}
if (best_len != -1) {
// We can respond, so we do (Greedy)
ties[best_len]--;
ties[x]++;
prev_ignored = false;
} else {
// Must ignore
if (prev_ignored) return false; // Failed: ignored two in a row
prev_ignored = true;
}
}
return true;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(NULL);
int n;
if (!(cin >> n)) return 0;
vector<int> A(n);
for (int i = 0; i < n; ++i) cin >> A[i];
int low = 1, high = n, ans = n;
while (low <= high) {
int mid = low + (high - low) / 2;
if (check(mid, n, A)) {
ans = mid;
high = mid - 1;
} else {
low = mid + 1;
}
}
cout << ans << endl;
return 0;
}
