USACO_DEC2015_PLATINUM HighCardLowCard

/*#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx,avx2,fma")*/
// only when really needed

/* GNU G++17 7.3.0: No long long for faster code
   GNU G++17 9.2.0 (64 bit, msys 2): Long long only for faster code */

#include <bits/stdc++.h>

#define for1(i,a,b) for (int i = a; i <= b; i++)
#define for2(i,a,b) for (int i = a; i >= b; i--)
#define int long long

#define sz(a) (int)a.size()
#define pii pair<int,int>

/*
__builtin_popcountll(x) : Number of 1-bit
__builtin_ctzll(x) : Number of trailing 0
*/

#define PI 3.1415926535897932384626433832795
#define INF 1000000000000000000
#define MOD 1000000007
#define MOD2 1000000009
#define EPS 1e-6

using namespace std;

int n, ans;
int a[50005], pre[50005], suf[50005];
bool f[100005];
set<int> se;

signed main() {

    ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);

    freopen("cardgame.in", "r", stdin);
    freopen("cardgame.out", "w", stdout);

    cin >> n;
    for1(i,1,n) {
        cin >> a[i];
        f[a[i]] = true;
    }

    for1(i,1,n * 2) if (!f[i]) se.insert(i);
    for1(i,1,n) {
        auto it = se.lower_bound(a[i]);
        pre[i] = pre[i - 1];
        if (it != se.end()) {
            pre[i]++;
            se.erase(it);
        }
    }
    se.clear();

    for1(i,1,n * 2) if (!f[i]) se.insert(i);
    for2(i,n,1) {
        auto it = se.lower_bound(a[i]);
        suf[i] = suf[i + 1];
        if (it != se.begin()) {
            suf[i]++;
            se.erase(prev(it));
        }
    }
    se.clear();

    for1(i,0,n) ans = max(ans, pre[i] + suf[i + 1]);
    cout << ans;

}