#include<bits/stdc++.h>
#include<bits/extc++.h>
#define int long long
#define inf 0x3f3f3f3f3f3f3f3f
#define maxn 1234567
#define eps 1e-7
#define mod 1000000007
#define Mod 998244353
#define f(i,a,b) for(int i=a;i<=b;i++)
#define r(i,a,b) for(int i=a;i>=b;i--) 
#define fx(i,a,b,x) for(int i=a;i<=b;i+=x)
#define rx(i,a,b,x) for(int i=a;i>=b;i-=x) 
using namespace std;
using namespace __gnu_cxx;
using namespace __gnu_pbds;
int T;
char c,ans;
map<char,int>mp;
int cnt,Cnt;
void solve(){
	f(i,1,13)cin>>c,mp[c]++;
	for(auto [k,v]:mp){
		if(v>2){
			printf("Impossible\n");
			return;
		}
		if(v==2)cnt++;
		else Cnt++,ans=k;
	}
	if(cnt==6&&Cnt==1)printf("%c\n",ans);
	else printf("Impossible\n");
}
signed main(){
	T=1;
//	scanf("%lld",&t);
	while(T--)solve();
	return 0;
}