def checkfile(x):


    L = []
    while True:
        x = raw_input('Please write the file you want to comapre : ')
        if x == 'Escape':
            break
        fin = open(x, 'r')
        fin = fin.readlines()
        L.append(fin)
        l_intersection = L[0]
        for l in L:
            l_intersection = set(l_intersection).intersection(l)
        print l_intersection

print checkfile('a')