#include <bits/stdc++.h>
using namespace std;
 
int data[200010];
 
int main() {
	// input: 
	// ori: 3 4 5
	// aux: 2 1
	// move = 0
	int n;
	cin >> n;
	stack<int> ori, aux;
	for(int i = 0; i < 2*n; i++)
		cin >> data[i];
	for(int i = 2*n-1; i >= 0; i--) {
		ori.push(data[i]);
	}
	int move = 0;
	while(!ori.empty()) {
		move++;
		if(!aux.empty() && aux.top() == ori.top()) {
			aux.pop();
			ori.pop();
		}
		else {
			aux.push(ori.top());
			ori.pop();
		} 
	}
	if(!aux.empty())
		cout << "impossible" << endl;
	else
		cout << move << endl;
	return 0;
}

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

Mwo1IDUgNSA1IDUgNQoKCg==

3
5 5 5 5 5 5