#include <stdio.h>
#include <string.h>

int main(){
    char str[13];
    int flag[26] = {0};
    int i, i_cnt, cnt = 0, cnt1 = 0;

    for(i = 0; i < 13; i++){
        scanf("%c", &str[i]);
        flag[str[i] - 'a']++;
    }
    for(i = 0; i < 26; i++){
        if(flag[i] == 1){
            i_cnt = i;
            cnt++;
            cnt1++;
        }else if(flag[i] == 2){
            cnt++;
        }
        if(i == 25 && cnt != 7){
            printf("Impossible\n");
        }
    }
    if(cnt == 7 && cnt1 == 1){
        printf("%c\n", 'a' + i_cnt);
    }else{
        printf("Impossible\n");
    }
    return 0;
}