Language: C++

/**
 *	author: nok0
 *	created: 2020.09.13 22:45:14
**/
#ifdef LOCAL
#define _GLIBCXX_DEBUG
#endif
#include<bits/stdc++.h>
using namespace std;
#include<atcoder/all>
using namespace atcoder;
#pragma region Macros
#define ll long long
#define ld long double
#define FOR(i,l,r) for(int i=(l);i<(r);++i)
#define REP(i,n) FOR(i,0,n)
#define REPS(i,n) FOR(i,1,n+1)
#define RFOR(i,l,r) for(int i=(l);i>=(r);--i)
#define RREP(i,n) RFOR(i,n-1,0)
#define RREPS(i,n) RFOR(i,n,1)
#define pb push_back
#define eb emplace_back
#define SZ(x) ((int)(x).size())
#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
template<class T = int> using V = vector<T>;
template<class T = int> using VV = V<V<T>>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define VEC(type, name, size)\
    V<type> name(size);\
    IN(name)
#define VVEC(type, name, h, w)\
    VV<type> name(h, V<type>(w));\
    IN(name)
#define INT(...)\
    int __VA_ARGS__;\
    IN(__VA_ARGS__)
#define LL(...)\
    ll __VA_ARGS__;\
    IN(__VA_ARGS__)
#define STR(...)\
    string __VA_ARGS__;\
    IN(__VA_ARGS__)
#define CHAR(...)\
    char __VA_ARGS__;\
    IN(__VA_ARGS__)
#define DOUBLE(...)\
    DOUBLE __VA_ARGS__;\
    IN(__VA_ARGS__)
#define LD(...)\
    LD __VA_ARGS__;\
    IN(__VA_ARGS__)
template <class T> void scan(T a) { cin >> a; }
void scan(int &a) { cin >> a; }
void scan(long long &a) { cin >> a; }
void scan(char &a) { cin >> a; }
void scan(double &a) { cin >> a; }
void scan(long double &a) { cin >> a; }
void scan(char a[]) { scanf("%s", a); }
void scan(string &a) { cin >> a; }
template <class T> void scan(V<T> &);
template <class T, class L> void scan(pair<T, L> &);
template <class T> void scan(V<T> &a) { for(auto &i : a) scan(i); }
template <class T, class L> void scan(pair<T, L> &p){ scan(p.first); scan(p.second); }
template <class T> void scan(T &a) { cin >> a; }
void IN() {}
template <class Head, class... Tail> void IN(Head &head, Tail &... tail) { scan(head); IN(tail...); }
template <class T> inline void print(T x){ cout << x << '\n';}
template <typename T1, typename T2>
istream &operator>>(istream &is, pair<T1, T2> &p){
    is >> p.first >> p.second;
    return is;
}
template <typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> &p){
    os << p.first << " " << p.second;
    return os;
}
template <class T>
ostream &operator<<(ostream &os, const V<T> &v){
    REP(i, SZ(v)){
        if(i) os << " ";
        os << v[i];
    }
    return os;
}
//debug
template <typename T>
void view(const V<T> &v){
    cerr << "{ ";
    for(const auto &e : v){
        cerr << e << ", ";
    }
    cerr << "\b\b }";
}
template <typename T>
void view(const VV<T> &vv){
    cerr << "{\n";
    for(const auto &v : vv){
        cerr << "\t";
        view(v);
        cerr << "\n";
    }
    cerr << "}";
}
template <typename T, typename U>
void view(const V<pair<T, U>> &v){
    cerr << "{\n";
    for(const auto &c : v) cerr << "\t(" << c.first << ", " << c.second << ")\n";
    cerr << "}";
}
template <typename T, typename U>
void view(const map<T, U> &m){
    cerr << "{\n";
    for(auto &t : m) cerr << "\t[" << t.first << "] : " << t.second << "\n";
    cerr << "}";
}
template <typename T, typename U>
void view(const pair<T, U> &p){ cerr << "(" << p.first << ", " << p.second << ")";}
template <typename T>
void view(const set<T> &s) {
    cerr << "{ ";
    for(auto &t : s) {
        view(t);
        cerr << ", ";
    }
    cerr << "\b\b }";
}
template <typename T>
void view(T e) { cerr << e;}
#ifdef LOCAL
void debug_out() {}
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    view(H);
    cerr << ", ";
    debug_out(T...);
}
#define debug(...)                                           \
    do{                                                     \
        cerr << __LINE__ << " [" << #__VA_ARGS__ << "] : ["; \
        debug_out(__VA_ARGS__);                              \
        cerr << "\b\b]\n";                                   \
    } while(0)
#else
#define debug(...) (void(0))
#endif
template <class T> V<T> press(V<T> &x){ 
    V<T> res = x;
    sort(all(res));
    res.erase(unique(all(res)), res.end());
    REP(i, SZ(x)){
        x[i] = lower_bound(all(res), x[i]) - res.begin();
    }
    return res;
}
template<class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true; }return false; }
template<class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true; }return false; }
inline void Yes(bool b = true) {cout << (b ? "Yes" : "No") << '\n';}
inline void YES(bool b = true) {cout << (b ? "YES" : "NO") << '\n';}
inline void err(bool b = true) {if(b) {cout << -1 << '\n'; exit(0);}}
template<class T> inline void fin(bool b = true, T e = 0) {if(b) {cout << e << '\n'; exit(0);}}
template<class T> T divup(T x, T y) {return (x+(y-1))/y;}
template <typename T> T pow(T a, long long n, T e = 1) {T ret = e; while (n) {if (n & 1) ret *= a; a *= a; n >>= 1; } return ret; }
const int inf = 1e9;
const ll INF = 1e18;
#pragma endregion

// Graph Template
struct Edge{
    ll to,cost;
    Edge(ll to,ll cost):to(to),cost(cost){}
    bool operator < (const Edge& a) const{
        return cost < a.cost;
    }
};
using Graph = VV<>;
using WGraph = VV<Edge>;

void Read_Graph(Graph &g, int m, bool directed = false){
    REP(i, m){
        LL(u, v); u--; v--;
        g[u].pb(v);
        if(!directed) g[v].pb(u);
    }
}
void Read_Tree(Graph &g, bool directed = false) {Read_Graph(g, SZ(g) - 1, directed);}

void Read_Graph(WGraph &g, int m, bool directed = false){
    REP(i, m){
        LL(u, v, c); u--; v--;
        g[u].pb({v, c});
        if(!directed) g[v].pb({u, c});
    }
}
void Read_Tree(WGraph &g, bool directed = false) {Read_Graph(g, SZ(g) - 1, directed);}

pair<int, int> dfs(const Graph &G, int u, int par) {  // 最遠点間距離と最遠点を求める
    pair<int, int> ret = make_pair(0, u);
    for (auto e : G[u]) {
        if (e == par) continue;
        auto next = dfs(G, e, u);
        next.first += 1;
        ret = max(ret, next);
    }
    return ret;
}
 
int tree_diamiter(const Graph &G) {
    pair<int, int> p = dfs(G, 0, -1);
    pair<int, int> q = dfs(G, p.second, -1);
    return q.first;
}

int main(){
    INT(n);
    Graph G(n);
    Read_Tree(G);
    debug(tree_diamiter(G));
    if(tree_diamiter(G) % 3 != 1) print("First");
    else print("Second");
}

Compiler output:
a.cc:10:9: fatal error: atcoder/all: No such file or directory
   10 | #include<atcoder/all>
      |         ^~~~~~~~~~~~~
compilation terminated.