const lines = require('fs')
  .readFileSync('/dev/stdin', 'utf-8')
  .trim().split('\n').values();

const S = lines.next().value;

const deck = Array(13).fill(0);

for (c of S) deck[c.charCodeAt(0) - 97]++;

if ((deck.reduce((a, x) => a + Number(x === 2), 0)) === 1) {
  deck.forEach((x, i) => {
    if (x === 0) {
      console.log(String.fromCharCode(i + 97));
    }
  });
  return;
}

if (deck.every(x => x === 1)) {
  deck.forEach((x, i) => {
    console.log(String.fromCharCode(i + 97));
  });
  return;
}

console.log('Impossible');