#include <bits/stdc++.h>
#define int long long
#define MAXN 1000010
using namespace std;
int T,last,l,r,len;
string opt;
map<string,int> a,b;
string Q[MAXN],x;
void solve()
{
	cin>>T;
	while(T--)
	{
		cin>>opt;
		if(opt=="start")
		{
			while(last)
			{
				last--;
				while(!a[Q[l]]) l++;
				Q[r++]=Q[l];
				b[Q[l]]=0;
				l++;
			}
			if(len==1)
			{
				last=1;
				while(!a[Q[l]]) l++;
				cout<<Q[l]<<'\n';
				b[Q[l]]=1;
			}
			else if(len>=2)
			{
				last=2;
				while(!a[Q[l]]) l++;
				cout<<Q[l]<<' ';
				b[Q[l]]=1;
				int tmp=l+1;
				while(!a[Q[tmp]]) tmp++;
				cout<<Q[tmp]<<'\n';
				b[Q[tmp]]=1;
			}
			else cout<<"Error"<<'\n';
		}
		else if(opt=="arrive")
		{
			cin>>x;
			if(a[x])
				cout<<"Error"<<'\n';
			else
			{
				Q[r++]=x;
				a[x]=1;
				len++;
				cout<<"OK"<<'\n';
			}
		}
		else
		{
			cin>>x;
			if(b[x]||!a[x])
				cout<<"Error"<<'\n';
			else
			{
				a[x]=0;
				len--;
				cout<<"OK"<<'\n';
			}
		}
	}
}
signed main()
{
	ios::sync_with_stdio(0);
	int _T=1;//cin>>_T;
	while(_T--) solve();
	return 0;
}