#include <bits/stdc++.h> using namespace std; #define int long long using ll=long long; using vi=vector<int>; using pii=pair<int,int>; #define ALL(c) begin(c),end(c) #define RALL(c) rbegin(c),rend(c) #define ITR(i,b,e) for(auto i=(b);i!=(e);++i) #define FORE(x,c) for(auto &x:c) #define REPF(i,a,n) for(int i=a,i##len=(int)(n);i<i##len;++i) #define REP(i,n) REPF(i,0,n) #define REPR(i,n) for(int i=(int)(n);i>=0;--i) #define SZ(c) ((int)c.size()) #define CONTAIN(c,x) (c.find(x)!=end(c)) #define OUTOFRANGE(y,x,h,w) (y<0||x<0||y>=h||x>=w) #define dump(...) const int DX[9]={0,1,0,-1,1,1,-1,-1,0},DY[9]={-1,0,1,0,-1,1,1,-1,0}; #define INF (1001001001) #define INFLL (1001001001001001001ll) template<class T> ostream& operator << (ostream &os,const vector<T> &v) { ITR(i,begin(v),end(v)) os<<*i<<(i==end(v)-1?"":" "); return os; } template<class T> istream& operator >> (istream &is,vector<T> &v) { ITR(i,begin(v),end(v)) is>>*i; return is; } template<class T> istream& operator >> (istream &is, pair<T,T> &p) { is>>p.first>>p.second; return is; } template <class T> bool chmax(T &a,const T &b) {if(a<b) {a=b;return 1;} return 0;} template <class T> bool chmin(T &a,const T &b) {if(b<a) {a=b;return 1;} return 0;} template <class T> using heap = priority_queue<T,vector<T>,greater<T>>; struct before_main_function { before_main_function() { #ifdef int #undef INF #define INF INFLL #define stoi stoll #endif cin.tie(0);ios::sync_with_stdio(false); cout<<setprecision(15)<<fixed; #define endl "\n" } } before_main_function; //------------------8<------------------------------------8<-------------------- map<char,int> m; signed main() { string s; cin>>s; FORE(x,s) m[x]++; int cnt=0; char ans; FORE(p,m) { if(p.second==1) { cnt++; ans=p.first; } if(p.second>2) { cout<<"Impossible"<<endl; return 0; } } if(cnt>1) { cout<<"Impossible"<<endl; } else { cout<<ans<<endl; } return 0; }