Language: C++

#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
const int MX = 200005;
#define RET(ans) {cout<<ans<<endl;return 0;} 

    // 二次元ベクターの基本
    /*
    vector<vector<int>> dp; // 宣言
    dp.resize(n); // 1次元めの要素数決定
    dp[i].push_back(int); // プッシュバック
    rep(i,n){
        sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
    }
    */

    // 整数スキャン（複数）
    /*
    int x,y,z;
    scanf("%d%d%d",&x,&y,&z);

    // n個の整数のスキャン
    /*
    ll a[n] = {};
    rep(i,n){
        scanf("%lld",&a[i]);
    }
    */

    // 文字列スキャン
    /*
    string s; cin >> s;
    */

    // n個の文字列スキャン
    /*
    vector<string> slist;
    rep(i,n){
        string s; cin >> s;
        slist.push_back(s);
    }
    */

int main() {
    int n;
    cin >> n;


    ll ans = 0;
    
    cout << ans << endl;
    return 0;
}
 
 
#include <bits/stdc++.h>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) (x = max(x,y))
#define mins(x,y) (x = min(x,y))
#define limit(x,l,r) max(l,min(x,r))
#define lims(x,l,r) (x = max(l,min(x,r)))
#define isin(x,l,r) ((l) <= (x) && (x) < (r))
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcountll
#define uni(x) x.erase(unique(rng(x)),x.end())
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define show(x) cout<<#x<<" = "<<x<<endl;
#define PQ(T) priority_queue<T,v(T),greater<T> >
#define bn(x) ((1<<x)-1)
#define dup(x,y) (((x)+(y)-1)/(y))
#define newline puts("")
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
typedef long long int ll;
typedef unsigned uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vl;
typedef vector<P> vp;
inline int in() { int x; scanf("%d",&x); return x;}
template<typename T>inline istream& operator>>(istream&i,v(T)&v)
{rep(j,sz(v))i>>v[j];return i;}
template<typename T>string join(const v(T)&v)
{stringstream s;rep(i,sz(v))s<<' '<<v[i];return s.str().substr(1);}
template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
{if(sz(v))o<<join(v);return o;}
template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
{return i>>v.fi>>v.se;}
template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
{return o<<v.fi<<","<<v.se;}
template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
const double eps = 1e-10;
const ll LINF = 1001002003004005006ll;
const int INF = 1001001001;
#define dame { puts("-1"); return 0;}
#define yn {puts("Yes");}else{puts("No");}
const int MX = 200005;
#define RET(ans) {cout<<ans<<endl;return 0;} 

    // 二次元ベクターの基本
    /*
    vector<vector<int>> dp; // 宣言
    dp.resize(n); // 1次元めの要素数決定
    dp[i].push_back(int); // プッシュバック
    rep(i,n){
        sort(dp[i].begin(),dp[i].end()); // 二次元めを昇順ソート
    }
    */

    // 整数スキャン（複数）
    /*
    int x,y,z;
    scanf("%d%d%d",&x,&y,&z);

    // n個の整数のスキャン
    /*
    ll a[n] = {};
    rep(i,n){
        scanf("%lld",&a[i]);
    }
    */

    // 文字列スキャン
    /*
    string s; cin >> s;
    */

    // n個の文字列スキャン
    /*
    vector<string> slist;
    rep(i,n){
        string s; cin >> s;
        slist.push_back(s);
    }
    */

int main() {
    string x;
    cin >> x;
    if(x[0]=='A')cout << 'T' << endl;
    if(x[0]=='T')cout << 'A' << endl;
    if(x[0]=='G')cout << 'C' << endl;
    if(x[0]=='C')cout << 'G' << endl;
    return 0;
}
 
 


Compiler output:
a.cc:138:12: error: redefinition of 'int in()'
  138 | inline int in() { int x; scanf("%d",&x); return x;}
      |            ^~
a.cc:35:12: note: 'int in()' previously defined here
   35 | inline int in() { int x; scanf("%d",&x); return x;}
      |            ^~
a.cc:139:37: error: redefinition of 'template<class T> std::istream& operator>>(std::istream&, std::vector<_Tp>&)'
  139 | template<typename T>inline istream& operator>>(istream&i,v(T)&v)
      |                                     ^~~~~~~~
a.cc:36:37: note: 'template<class T> std::istream& operator>>(std::istream&, std::vector<_Tp>&)' previously declared here
   36 | template<typename T>inline istream& operator>>(istream&i,v(T)&v)
      |                                     ^~~~~~~~
a.cc:141:28: error: redefinition of 'template<class T> std::string join(const std::vector<_Tp>&)'
  141 | template<typename T>string join(const v(T)&v)
      |                            ^~~~
a.cc:38:28: note: 'template<class T> std::string join(const std::vector<_Tp>&)' previously declared here
   38 | template<typename T>string join(const v(T)&v)
      |                            ^~~~
a.cc:143:37: error: redefinition of 'template<class T> std::ostream& operator<<(std::ostream&, const std::vector<_Tp>&)'
  143 | template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
      |                                     ^~~~~~~~
a.cc:40:37: note: 'template<class T> std::ostream& operator<<(std::ostream&, const std::vector<_Tp>&)' previously declared here
   40 | template<typename T>inline ostream& operator<<(ostream&o,const v(T)&v)
      |                                     ^~~~~~~~
a.cc:145:50: error: redefinition of 'template<class T1, class T2> std::istream& operator>>(std::istream&, std::pair<_T1, _T2>&)'
  145 | template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
      |                                                  ^~~~~~~~
a.cc:42:50: note: 'template<class T1, class T2> std::istream& operator>>(std::istream&, std::pair<_T1, _T2>&)' previously declared here
   42 | template<typename T1,typename T2>inline istream& operator>>(istream&i,pair<T1,T2>&v)
      |                                                  ^~~~~~~~
a.cc:147:50: error: redefinition of 'template<class T1, class T2> std::ostream& operator<<(std::ostream&, const std::pair<_T1, _T2>&)'
  147 | template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
      |                                                  ^~~~~~~~
a.cc:44:50: note: 'template<class T1, class T2> std::ostream& operator<<(std::ostream&, const std::pair<_T1, _T2>&)' previously declared here
   44 | template<typename T1,typename T2>inline ostream& operator<<(ostream&o,const pair<T1,T2>&v)
      |                                                  ^~~~~~~~
a.cc:149:31: error: redefinition of 'template<class T> ll suma(const std::vector<_Tp>&)'
  149 | template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
      |                               ^~~~
a.cc:46:31: note: 'template<class T> ll suma(const std::vector<_Tp>&)' previously declared here
   46 | template<typename T>inline ll suma(const v(T)& a) { ll res(0); for (auto&& x : a) res += x; return res;}
      |                               ^~~~
a.cc:150:14: error: redefinition of 'const double eps'
  150 | const double eps = 1e-10;
      |              ^~~
a.cc:47:14: note: 'const double eps' previously defined here
   47 | const double eps = 1e-10;
      |              ^~~
a.cc:151:10: error: redefinition of 'const ll LINF'
  151 | const ll LINF = 1001002003004005006ll;
      |          ^~~~
a.cc:48:10: note: 'const ll LINF' previously defined here
   48 | const ll LINF = 1001002003004005006ll;
      |          ^~~~
a.cc:152:11: error: redefinition of 'const int INF'
  152 | const int INF = 1001001001;
      |           ^~~
a.cc:49:11: note: 'const int INF' previously defined here
   49 | const int INF = 1001001001;
      |           ^~~
a.cc:155:11: error: redefinition of 'const int MX'
  155 | const int MX = 200005;
      |           ^~
a.cc:52:11: note: 'const int MX' previously defined here
   52 | const int MX = 200005;
      |           ^~
a.cc:195:5: error: redefinition of 'int main()'
  195 | int main() {
      |     ^~~~
a.cc:92:5: note: 'int main()' previously defined here
   92 | int main() {
      |     ^~~~