const inputStr = require("fs").readFileSync("/dev/stdin", "utf8")
const alls = [ 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm' ];
const inputs = inputStr.split("").filter(v => v !== "\n");
const removedDuplicates = Array.from(new Set(inputs));
if (removedDuplicates.length === 13) {
    console.log(alls.join("\n"))
} else if (removedDuplicates.length === 12) {
    for (let c of alls) {
        if (!(removedDuplicates.includes(c))) {
            console.log(c)
            return;
        }
    }
} else {
    console.log("Impossible")
}