#include<bits/stdc++.h>
using namespace std;
int n,c=-1,sum,ok=1;
char ch[8001];
int main(){
	scanf("%d%s",&n,ch);
	int j=n-1;
	for(int i=0;i<=j-1;i++){
		for(int k=j;k>=i;k--){
			if(k==i){
				if(n%2==0||c!=-1){
					ok=0;
					break;
				}
				c=1;
				sum+=n/2-i;
				break;
			}
			if(ch[k]==ch[i]){
				for(int t=k;t<=j-1;t++){
				ch[t]=ch[t+1];	
				}
				sum+=j-k;
				j--;
				break;
			}
		}
		if(!ok) break;
	}
	if(!ok) printf("Impossible");
	else cout<<sum;


	return 0;
}

/**************************************************************
	Problem: 1842
	User: chenpengxi
	Language: C++
	Result: Accepted
	Time:54 ms
	Memory:2084 kb
****************************************************************/