# -*- coding: utf-8 -*-



def solve(input):
    chain = {}
    l = input.split()
    s = set([])
    for w in l:
        if w.isdigit() :
            continue

        for i in xrange(len(w)) :
            s.add(w[i])
            if i + 1< len(w) :
                chain[w[i]] = w[i + 1]

    if len(s) == 1 :
        return s.pop()

    if len(s) - 1 > len(chain) :
        return "-1"

    k = set(chain.keys())
    v = set(chain.values())

    first = ((k ^ v) & k).pop()
    end = ((k ^ v) & v).pop()

    i = first
    r = first

    while(i != end):
        r += chain[i]
        i = chain[i]

    return r
    pass

if __name__ == "__main__":
    print solve(raw_input())