Kuchiguse

def solve(n, data):
    n = int(n)
    strs = [s[::-1] for s in str(data).strip().split(',')]
    common = strs[0]
    for s in strs[1:]:
        new = ''
        for a, b in zip(common, s):
            if a == b: new += a
            else: break
        common = new
    print(common[::-1] if common else 'nai')