#include <algorithm>
#include <bitset>
#include <cassert>
#include <cctype>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <iomanip>
#include <iterator>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

const double EPS = 1e-9;
const double PI=acos(-1.0);

#define    READ(f) 	         freopen(f, "r", stdin)
#define    WRITE(f)   	     freopen(f, "w", stdout)

#define    INF   999999999999999
#define    pii 	 pair<int, int>
#define    ff 	 first
#define    ss 	 second
#define    ii	 long long int
#define    uii 	 unsigned long long int
#define    ui    unsigned int
#define    MOD   1000000007

template< class T > inline T _abs(T n) { return ((n) < 0 ? -(n) : (n)); }
template< class T > inline T _max(T a, T b) { return (!((a)<(b))?(a):(b)); }
template< class T > inline T _min(T a, T b) { return (((a)<(b))?(a):(b)); }
template< class T > inline T _swap(T &a, T &b) { a=a^b;b=a^b;a=a^b;}
template< class T > inline T gcd(T a, T b) { return (b) == 0 ? (a) : gcd((b), ((a) % (b))); }
template< class T > inline T lcm(T a, T b) { return ((a) / gcd((a), (b)) * (b)); }


//******************DELETE****************

#ifdef rafiul41
     #define debug(args...) {cerr<<"Debug: "; dbg,args; cerr<<endl;}
#else
    #define debug(args...)  // Just strip off all debug tokens
#endif

struct debugger{
    template<typename T> debugger& operator , (const T& v){
        cerr<<v<<" ";
        return *this;
    }
}dbg;

int bitOn(int N,int pos)
{
    return N=N | (1<<pos);
}
int bitOff(int N,int pos)
{
    return N=N & ~(1<<pos);
}
bool bitCheck(int N,int pos)
{
    return (bool)(N & (1<<pos));
}

///*******************************************************************************************

typedef tree<
int,
null_type,
less<int>,
rb_tree_tag,
tree_order_statistics_node_update>
ordered_set;

ordered_set s;

int main()
{
#ifdef rafiul41

//    READ("in.txt");
//    WRITE("out.txt");

#endif // rafiul41
    int q;
    scanf("%d",&q);
    s.clear();
    for(int i=0;i<q;i++)
    {
        int x;
        char z[2];
        scanf("%s %d",z,&x);
        if(z[0]=='I')
        {
            s.insert(x);
        }
        else if(z[0]=='C')
        {
            //cout<<s.order_of_key(x)<<endl;
            printf("%d\n",s.order_of_key(x));
        }
        else if(z[0]=='K')
        {
            x--;
            if(s.find_by_order(x)==s.end())
            {
                //cout<<"invalid"<<endl;
                printf("invalid\n");
            }
            else
            {
                printf("%d\n",*s.find_by_order(x));
                //cout<<*s.find_by_order(x)<<endl;
            }
        }
        else
        {
            s.erase(x);
        }
    }
    return 0;
}