#include <iostream>
#include <set>
#include <algorithm>
#include <cassert>

using namespace std;

set<int> s;

void insert(int key)
{
    s.insert(key);
}

void remove(int key)
{
    s.erase(key);
}

int closest(int key)
{
    assert(s.begin() != s.end());

    auto it = lower_bound(s.begin(), s.end(), key);

    if (it == s.end()) return *(--it);
    if (it == s.begin()) return *it;

    int a = *it;
    int b = *(--it);

    if (abs(a - key) < abs(b - key)) {
        return a;
    }

    return b;
}