from collections.abc import Iterable
from enum import IntEnum, auto
from functools import cache
from unittest import case


class E(IntEnum):
    S = 0
    X_Y = auto()
    Y_X = auto()

    def nexts(self):
        match self:
            case E.S:return [E.X_Y, E.Y_X]
            case E.X_Y:return [E.X_Y, E.Y_X]
            case E.Y_X:return [E.X_Y, E.Y_X]

        assert False

    @staticmethod
    @cache
    def states():
        res = []
        for fm in E:
            for to in fm.nexts():
                res.append((fm, to))

        return res


def state_dp(xs: Iterable, op, e, init: dict, *, is_reset=True):
    dp = [e] * len(E)
    for k, v in init.items():
        dp[k] = v

    for x in xs:
        pp = [e] * len(E) if is_reset else dp.copy()
        dp, pp = pp, dp
        for fm, to in E.states():
            if not is_valid(to, pp[fm], x): continue
            dp[to] = op(to, dp[to], fm, pp[fm], x)

    return dp


def is_valid(to: E, fm_v, v) -> bool:
    return True


def op(to: E, to_v, fm: E, fm_v, v):
    s, e = fm_v  # (スコア, 選択要素)
    x, y = v

    if x == y:
        s += x

    match to:
        case E.X_Y:
            if x == e: s += x
            return max(to_v, (s, y))

        case E.Y_X:
            if y == e: s += y
            return max(to_v, (s, x))

    assert False


N = int(input())
xs = []
for _ in range(N):
    X, Y = map(int, input().split())
    xs.append((X, Y))

dp = state_dp(xs, op, (0, -1), {E.S: (0, -1)})
ans = max(s for s, _ in dp)
print(ans)