Language: C++

// #define _CRT_SECURE_NO_WARNINGS
// #define _USE_MATH_DEFINES	// M_PI=3.1415...
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stack>
#include <cmath>
#include <cstdlib>
#include <functional>
#include <locale>
#include <cctype>
#include <sstream>
#include <iomanip>	// 20桁出力 cout << setprecision(20) << double;
#include <queue>
#include <bitset>	// bitset<8> bs1(131uL); // 10000011 bs1[0]は1 01stringからビット集合生成可
#include <numeric>
#include <random>
using namespace std;
 
typedef long long LL;
typedef pair<LL, LL> P;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VL;
typedef vector<VL> VVL;				// VVL v(100,VL(100,0));
typedef vector<pair<LL, LL> > VP;
typedef vector<string> VS;
typedef map<int, int> MAPII;
typedef multimap<int, char, greater<int> > MuMAPIC;
typedef multimap<int, string, greater<int> > MuMIS;
typedef pair<LL, pair<P, P> > PP;
 
#define INF 999999999999999997		// 少し加算したらオーバーフローする
#define MP make_pair
#define FAST_IO  cin.tie(0); ios::sync_with_stdio(false);
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FORL(i,a,b) for(LL i=(a);i<(b);i++)
#define FOR_REV(i,a,b) for(int i=(a);i>=(b);i--)
#define foreach_it(type,it,c) for(type::iterator it=c.begin(),c_end=c.end();it!=c_end;++it)
#define FOR_ITR(d) for(auto itr=d.begin(),d_end=d.end();itr!=d_end;++itr)	// C++11
#define O(s) cout<<s<<endl;
 
#define DUMP_VVI(b) if(dbgF){FOR(i,0,b.size()){FOR(j,0,b[i].size())printf("%d ",b[i][j]);puts("");}}
#define D_OUT(str,value) if(dbgF){cout<<str<<" : "<<value<<endl;}
 
template<class T>T IN() { T d; cin >> d; return d; }
 
LL gcd(LL a, LL b) { return (b > 0) ? gcd(b, a%b) : a; } // 最大公約数（Greatest Common Divisor）
LL lcm(LL a, LL b) { return a / gcd(a, b) * b; } // 最小公倍数（Least Common Multiple）
bool uruu(LL Y) { return (((Y % 4 == 0 && Y % 100 != 0) || Y % 400 == 0) ? true : false); } // うるう年判定
string Replace(string S, string from, string to) { auto p = S.find(from); while (p != string::npos) { S.replace(p, from.size(), to); p = S.find(from, p + to.size()); }return S; }
 
int dx[4] = { 0,1,0,-1 };
int dy[4] = { 1,0,-1,0 };
int dxx[8] = { 0,1,1,1,0,-1,-1,-1 };
int dyy[8] = { 1,1,0,-1,-1,-1,0,1 };
 
// -------------------------------------------------------------------------------
 
int main() {
	FAST_IO;
	bool dbgF = true; // for D_OUT(str, value)  ... cout<< str <<" : "<< value <<endl;
 
	LL n;
	cin >> n;
	LL ans = n / 3;
	if (n < 3)ans = 0;
	O(ans);
 
	return 0;
}
// #define _CRT_SECURE_NO_WARNINGS
// #define _USE_MATH_DEFINES	// M_PI=3.1415...
#include <iostream>
#include <cstdio>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stack>
#include <cmath>
#include <cstdlib>
#include <functional>
#include <locale>
#include <cctype>
#include <sstream>
#include <iomanip>	// 20桁出力 cout << setprecision(20) << double;
#include <queue>
#include <bitset>	// bitset<8> bs1(131uL); // 10000011 bs1[0]は1 01stringからビット集合生成可
#include <numeric>
#include <random>
using namespace std;

typedef long long LL;
typedef pair<LL, LL> P;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<LL> VL;
typedef vector<VL> VVL;				// VVL v(100,VL(100,0));
typedef vector<pair<LL, LL> > VP;
typedef vector<string> VS;
typedef map<int, int> MAPII;
typedef multimap<int, char, greater<int> > MuMAPIC;
typedef multimap<int, string, greater<int> > MuMIS;
typedef pair<LL, pair<P, P> > PP;

#define INF 999999999999999997		// 少し加算したらオーバーフローする
#define MP make_pair
#define FAST_IO  cin.tie(0); ios::sync_with_stdio(false);
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define FORL(i,a,b) for(LL i=(a);i<(b);i++)
#define FOR_REV(i,a,b) for(int i=(a);i>=(b);i--)
#define foreach_it(type,it,c) for(type::iterator it=c.begin(),c_end=c.end();it!=c_end;++it)
#define FOR_ITR(d) for(auto itr=d.begin(),d_end=d.end();itr!=d_end;++itr)	// C++11
#define O(s) cout<<s<<endl;

#define DUMP_VVI(b) if(dbgF){FOR(i,0,b.size()){FOR(j,0,b[i].size())printf("%d ",b[i][j]);puts("");}}
#define D_OUT(str,value) if(dbgF){cout<<str<<" : "<<value<<endl;}

template<class T>T IN() { T d; cin >> d; return d; }

LL gcd(LL a, LL b) { return (b > 0) ? gcd(b, a%b) : a; } // 最大公約数（Greatest Common Divisor）
LL lcm(LL a, LL b) { return a / gcd(a, b) * b; } // 最小公倍数（Least Common Multiple）
bool uruu(LL Y) { return (((Y % 4 == 0 && Y % 100 != 0) || Y % 400 == 0) ? true : false); } // うるう年判定
string Replace(string S, string from, string to) { auto p = S.find(from); while (p != string::npos) { S.replace(p, from.size(), to); p = S.find(from, p + to.size()); }return S; }

int dx[4] = { 0,1,0,-1 };
int dy[4] = { 1,0,-1,0 };
int dxx[8] = { 0,1,1,1,0,-1,-1,-1 };
int dyy[8] = { 1,1,0,-1,-1,-1,0,1 };

// -------------------------------------------------------------------------------

int main() {
	FAST_IO;
	bool dbgF = true; // for D_OUT(str, value)  ... cout<< str <<" : "<< value <<endl;

	string s1,s2,s3;
	cin>>s1>>s2>>s3;
	cout<<s1[0]<<s2[1]<<s3[2];<<endl;
	
	return 0;
}


Compiler output:
a.cc:125:20: error: redefinition of 'template<class T> T IN()'
  125 | template<class T>T IN() { T d; cin >> d; return d; }
      |                    ^~
a.cc:50:20: note: 'template<class T> T IN()' previously declared here
   50 | template<class T>T IN() { T d; cin >> d; return d; }
      |                    ^~
a.cc:127:4: error: redefinition of 'LL gcd(LL, LL)'
  127 | LL gcd(LL a, LL b) { return (b > 0) ? gcd(b, a%b) : a; } // 最大公約数（Greatest Common Divisor）
      |    ^~~
a.cc:52:4: note: 'LL gcd(LL, LL)' previously defined here
   52 | LL gcd(LL a, LL b) { return (b > 0) ? gcd(b, a%b) : a; } // 最大公約数（Greatest Common Divisor）
      |    ^~~
a.cc:128:4: error: redefinition of 'LL lcm(LL, LL)'
  128 | LL lcm(LL a, LL b) { return a / gcd(a, b) * b; } // 最小公倍数（Least Common Multiple）
      |    ^~~
a.cc:53:4: note: 'LL lcm(LL, LL)' previously defined here
   53 | LL lcm(LL a, LL b) { return a / gcd(a, b) * b; } // 最小公倍数（Least Common Multiple）
      |    ^~~
a.cc:129:6: error: redefinition of 'bool uruu(LL)'
  129 | bool uruu(LL Y) { return (((Y % 4 == 0 && Y % 100 != 0) || Y % 400 == 0) ? true : false); } // うるう年判定
      |      ^~~~
a.cc:54:6: note: 'bool uruu(LL)' previously defined here
   54 | bool uruu(LL Y) { return (((Y % 4 == 0 && Y % 100 != 0) || Y % 400 == 0) ? true : false); } // うるう年判定
      |      ^~~~
a.cc:130:8: error: redefinition of 'std::string Replace(std::string, std::string, std::string)'
  130 | string Replace(string S, string from, string to) { auto p = S.find(from); while (p != string::npos) { S.replace(p, from.size(), to); p = S.find(from, p + to.size()); }return S; }
      |        ^~~~~~~
a.cc:55:8: note: 'std::string Replace(std::string, std::string, std::string)' previously defined here
   55 | string Replace(string S, string from, string to) { auto p = S.find(from); while (p != string::npos) { S.replace(p, from.size(), to); p = S.find(from, p + to.size()); }return S; }
      |        ^~~~~~~
a.cc:132:5: error: redefinition of 'int dx [4]'
  132 | int dx[4] = { 0,1,0,-1 };
      |     ^~
a.cc:57:5: note: 'int dx [4]' previously defined here
   57 | int dx[4] = { 0,1,0,-1 };
      |     ^~
a.cc:133:5: error: redefinition of 'int dy [4]'
  133 | int dy[4] = { 1,0,-1,0 };
      |     ^~
a.cc:58:5: note: 'int dy [4]' previously defined here
   58 | int dy[4] = { 1,0,-1,0 };
      |     ^~
a.cc:134:5: error: redefinition of 'int dxx [8]'
  134 | int dxx[8] = { 0,1,1,1,0,-1,-1,-1 };
      |     ^~~
a.cc:59:5: note: 'int dxx [8]' previously defined here
   59 | int dxx[8] = { 0,1,1,1,0,-1,-1,-1 };
      |     ^~~
a.cc:135:5: error: redefinition of 'int dyy [8]'
  135 | int dyy[8] = { 1,1,0,-1,-1,-1,0,1 };
      |     ^~~
a.cc:60:5: note: 'int dyy [8]' previously defined here
   60 | int dyy[8] = { 1,1,0,-1,-1,-1,0,1 };
      |     ^~~
a.cc:139:5: error: redefinition of 'int main()'
  139 | int main() {
      |     ^~~~
a.cc:64:5: note: 'int main()' previously defined here
   64 | int main() {
      |     ^~~~
a.cc: In function 'int main()':
a.cc:145:35: error: expected primary-expression before '<<' token
  145 |         cout<<s1[0]<<s2[1]<<s3[2];<<endl;
      |                                   ^~