cf271c wa

#include <assert.h>
#include <bits/stdc++.h>
using namespace std;
#define dbg(...) logger(#__VA_ARGS__, __VA_ARGS__)
template <typename... Args> void logger(string vars, Args &&... values)
{
    cerr << vars << " = ";
    string delim = "";
    (..., (cerr << delim << values, delim = ", "));
    cerr << endl;
}

template <class T> using mpq = priority_queue<T, vector<T>, greater<T>>;
using ll = long long;
using pii = pair<int, int>;

int n;
ll a[300003];

int main(int argc, char **argv)
{
    scanf("%d", &n);
    for (int i = 1; i <= n; ++i)
        scanf("%lld", a + i);

    sort(a + 1, a + 1 + n);
    int ans = 0;
    int expect = 1;

    for (int lb = 1, ub = n; lb <= ub; ++lb) {
        if (a[lb] == expect) {
            ++expect;
            ++ans;
        } else {
            while (lb + 1 <= ub) {
                ub -= 2;
                ++expect;
                ++ans;
                if (lb <= ub && a[lb] == expect) {
                    ++expect;
                    ++ans;
                    break;
                }
            }
        }
    }

    printf("%d\n", ans);
    return 0;
};