Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
INF = 10**18
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)


opposite = 0
max_dist = 0

def f(d,x):
  global max_dist,opposite
  dx = d[x]
  if dx > max_dist:
    opposite = x
    max_dist = dx
  for y in edge[x]:
    if d[y] == INF:
      d[y] = dx + 1
      f(d,y)


if N != 1:
  # 0からの距離
  d = [INF] * N
  d[0] = 0
  f(d,0)

  v = opposite
  d = [INF] * N
  d[v] = 0
  f(d,v)

  X = max_dist

  ans = 'First' if max_dist%3 != 1 else 'Second'
  print(ans)
else:
  print('First')


Fixed submission:
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)


def f(x):
  opposite = 0
  max_dist = 0
  # 最離点と距離とを返す
  d = [None]*N
  q = [(x,0)]
  while q:
    x,dx = q.pop()
    d[x] = dx
    if dx > max_dist:
      max_dist = dx
      opposite = x
    for y in edge[x]:
      if d[y] is None:
        q.append((y,dx+1))
  return opposite,max_dist

if N != 1:
  # 0からの距離
  v,_ = f(0)
  _,max_dist = f(v)

  ans = 'First' if max_dist%3 != 1 else 'Second'
  print(ans)
else:
  print('First')

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
INF = 10**18
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)

opposite = 0
max_dist = 0
def f(d,x):
  global max_dist,opposite
  dx = d[x]
  if dx > max_dist:
    opposite = x
    max_dist = dx
  for y in edge[x]:
    if d[y] == INF:
      d[y] = dx + 1
      f(d,y)

# 0からの距離
d = [INF] * N
d[0] = 0
f(d,0)

v = opposite
d = [INF] * N
d[v] = 0
f(d,v)

X = max_dist

ans = 'First' if max_dist%3 != 1 else 'Second'
print(ans)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)


def f(x):
  opposite = 0
  max_dist = 0
  # 最離点と距離とを返す
  d = [None]*N
  q = [(x,0)]
  while q:
    x,dx = q.pop()
    d[x] = dx
    if dx > max_dist:
      max_dist = dx
      opposite = x
    for y in edge[x]:
      if d[y] is None:
        q.append((y,dx+1))
  return opposite,max_dist

if N != 1:
  # 0からの距離
  v,_ = f(0)
  _,max_dist = f(v)

  ans = 'First' if max_dist%3 != 1 else 'Second'
  print(ans)
else:
  print('First')

Buggy python submission (verdict: Time Limit Exceeded):
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
INF = 10**18
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)

opposite = 0
max_dist = 0
def f(d,x):
  global max_dist,opposite
  dx = d[x]
  if dx > max_dist:
    opposite = x
    max_dist = dx
  for y in edge[x]:
    if d[y] == INF:
      d[y] = dx + 1
      f(d,y)

# 0からの距離
d = [INF] * N
d[0] = 0
f(d,0)

v = opposite
d = [INF] * N
d[v] = 0
f(d,v)

X = max_dist

ans = 'First' if max_dist%3 != 1 else 'Second'
print(ans)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)


def f(x):
  opposite = 0
  max_dist = 0
  # 最離点と距離とを返す
  d = [None]*N
  q = [(x,0)]
  while q:
    x,dx = q.pop()
    d[x] = dx
    if dx > max_dist:
      max_dist = dx
      opposite = x
    for y in edge[x]:
      if d[y] is None:
        q.append((y,dx+1))
  return opposite,max_dist

if N != 1:
  # 0からの距離
  v,_ = f(0)
  _,max_dist = f(v)

  ans = 'First' if max_dist%3 != 1 else 'Second'
  print(ans)
else:
  print('First')

Buggy python submission (verdict: Runtime Error):
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
INF = 10**18
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)

opposite = 0
max_dist = 0
def f(d,x):
  global max_dist,opposite
  dx = d[x]
  if dx > max_dist:
    opposite = x
    max_dist = dx
  for y in edge[x]:
    if d[y] == INF:
      d[y] = dx + 1
      f(d,y)

# 0からの距離
d = [INF] * N
d[0] = 0
f(d,0)

v = opposite
d = [INF] * N
d[v] = 0
f(d,v)

X = max_dist

ans = 'First' if max_dist%3 != 1 else 'Second'
print(ans)

Fixed submission:
import sys
sys.setrecursionlimit(10**6)
# グラフの最長経路が-1か-2かを選べる
N = int(input())
edge = [[] for _ in range(N)]
for _ in range(N-1):
  a,b = map(int,input().split())
  edge[a-1].append(b-1)
  edge[b-1].append(a-1)


def f(x):
  opposite = 0
  max_dist = 0
  # 最離点と距離とを返す
  d = [None]*N
  q = [(x,0)]
  while q:
    x,dx = q.pop()
    d[x] = dx
    if dx > max_dist:
      max_dist = dx
      opposite = x
    for y in edge[x]:
      if d[y] is None:
        q.append((y,dx+1))
  return opposite,max_dist

if N != 1:
  # 0からの距離
  v,_ = f(0)
  _,max_dist = f(v)

  ans = 'First' if max_dist%3 != 1 else 'Second'
  print(ans)
else:
  print('First')