n = int(input())
sp = []
c = 0
for i in range(n):
    b = input()
    sp.append(b)
sp.sort()
for i in sp:
    if sp.count(i) >= 2 and c == 0:
        print(i)
        c = 1
    if sp.count(i) < 2:
        c = 0

why you use variable 'c'