def solve():
    n = list(map(int, input().split()))[0]
    x = 0
    y = 0
    dp0 = 0
    dp1 = 0
    for _ in range(n):
        pdp0, pdp1 = dp0, dp1
        arr = list(map(int, input().split()))
        nx, ny = arr

        for _ in range(2):
            ndp00 = pdp0
            if y == nx:
                ndp00 += nx
            ndp01 = pdp1
            if x == nx:
                ndp01 += nx
            dp0 = max(ndp00, ndp01)
            if nx == ny:
                dp0 += nx
            x, y = y, x
            nx, ny = ny, nx
            dp0, dp1 = dp1, dp0
            pdp0, pdp1 = pdp1, pdp0
        x, y = nx, ny

    print(max(dp0, dp1))


if __name__ == '__main__':
    solve()