n = int(input())

dp = [[0, 0] for _ in range(n + 1)]
xy = []
for _ in range(n):
    x, y = map(int, input().split())
    xy.append((x, y))
xy.append((-1, -1))

for i in range(n):
    a = xy[i - 1]
    b = xy[i]
    t = b[0] * int(b[0] == b[1])
    for j in range(2):
        for k in range(2):
            c = t + b[k] * int(a[j ^ 1] == b[k])
            dp[i + 1][k] = max(dp[i + 1][k], dp[i][j] + c)

print(max(dp[-1]))