#include<bits/stdc++.h>
using namespace std;
int n,ans=0;
string s;
int main(){
    cin>>n>>s;
    while(s.size()>=2){
    	int m=s.size()-1;
    	char c=s[m];
    	int x=s.find(c);
    	if(x<m){
    	    ans+=x;
			s.erase(m,m);
			s.erase(x,x);	
    	}else{
    		if(m%2==0){
    			ans+=x-s.size()/2;
			    s.erase(x,x);
    		}else{
    			cout<<"Impossible";
    		} 
    	}
    }
    cout<<ans;
	return 0;
}
/**************************************************************
	Problem: 1842
	User: fhy
	Language: C++
	Result: Output Limit Exceed
****************************************************************/