MAX = 2 * 10 ** 5
n = int(input())
g = [[] for _ in range(MAX + 1)]
inv = [0] * (MAX + 1)
out = [0] * (MAX + 1)
s = set()
for i in range(n):
    h, w = map(int, input().split())
    s.add(h)
    s.add(w)
    g[h].append(w)
    inv[w] += 1
    out[h] += 1
f1 = 0
f2 = 0
for i in range(MAX + 1):
    if inv[i] + 1 == out[i]:
        f1 += 1
    elif out[i] + 1 == inv[i]:
        f2 += 1
    elif inv[i] == out[i]:
        continue
    else:
        print(0)
        exit(0)
if f1 == f2 == 0:
    print(len(s))
elif f1 == f2 == 1:
    print(1)
else:
    print(0)