import sys
def I(): return int(sys.stdin.readline().rstrip())
def MI(): return map(int,sys.stdin.readline().rstrip().split())
def LI(): return list(map(int,sys.stdin.readline().rstrip().split()))
def LI2(): return list(map(int,sys.stdin.readline().rstrip()))
def S(): return sys.stdin.readline().rstrip()
def LS(): return list(sys.stdin.readline().rstrip().split())
def LS2(): return list(sys.stdin.readline().rstrip())


N = I()

M = 2*10**5
in_deg = [0]*(M+1)
out_deg = [0]*(M+1)
for _ in range(N):
    H,W = MI()
    out_deg[H] += 1
    in_deg[W] += 1

a,b,c = 0,0,0
for i in range(1,M+1):
    if in_deg[i] == out_deg[i]:
        if in_deg[i]:
            a += 1
    elif in_deg[i] == out_deg[i]+1:
        b += 1
    elif in_deg[i]+1 == out_deg[i]:
        c += 1
    else:
        print(0)
        exit()

if b > 1 or c > 1 or b != c:
    print(0)
    exit()

if b:
    print(1)
else:
    print(a)