1.1

def pref(s):
    k = 0
    s1 = min(s, key=len)
    for i in range(len(s1)):
        k = 0
        s2 = s1[0:len(s1) - i]
        for j in range(len(s)):
            if s[j].find(s2) != -1:
                k += 1
        if k == len(s):
            return s2
    return ""


s = ["flower", "flow", "flight"]
print(pref(s))