const readline_module = require('readline');


const readline = () => (new Promise((resolve, reject) => {
  const rl = readline_module.createInterface(process.stdin);
  rl.on('line', line => {
    resolve(line);
    rl.close();
  });
}));


const message = (async () => {
  const str = await readline();
  const map = new Map();
  for (const char of str) {
    const item = map.get(char);
    map.set(char, item === undefined ? 1 : item + 1);
  }
  for (const [a, b] of map) {
    if (b === 1) return a;
  }
  return 'Impossible';

})();

(async () => {
  console.log(await message)
})();