#include <bits/stdc++.h>
using namespace std;

long long a[13], c;
static char s1[] = "Impossible", s2[] = "a\nb\nc\nd\ne\nf\ng\nh\ni\nj\nk\nl\nm";

int main(){
    for(int i = 13; --i >= 0; ){
        c = getchar() - 97;
        ++a[c];
    }
    c = -1;
    for(int i = 13; --i >= 0; ){
        if(!a[i])
            if(c >= 0)return 0 & puts(s1);
            else c = i;
    }
    if(c >= 0)cout << char(c + 97) << endl;
    else puts(s2);
    return 0;
}