#include <bits/stdc++.h>
using namespace std;
 
int main() {
	int n, move = 0;
	cin >> n;
	stack<int> ori, aux;
	for(int i = 0; i < 2*n; i++) {
		int tmp;
		cin >> tmp;
		ori.push(tmp);
	}
	while(!ori.empty()) {
		move++;
		if(!aux.empty() && aux.top() == ori.top()) {
			aux.pop();
			ori.pop();
			continue;
		}
		aux.push(ori.top());
		ori.pop();
	}
	if(!aux.empty())
		cout << "impossible" << endl;
	else
		cout << move << endl;
	return 0;
}

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

MwozIDIgMSAxIDMgMgo=

3
3 2 1 1 3 2