Language: C++

/*
    atcoder_begineer_c_112A       4246

   28/10/18
   by ahmed_drawy


*/


//#include <bits/stdc++.h>
#include <iostream>
using namespace std;
//#define push_back               pb;
//#define make_pair               mp;
#define lp(i,start , end)       for(int i = start ; i<end ; ++i)
#define Rlp(i,start , end)      for(int i = start ; i>end ; --i)
#define all(v)                   ((v).begin(),(v).end())
#define sz(v)                  (int)((v).size())
#define clr(v,d)                memset(v , d , sizeof(v))
#define isOn(S, j) (S & (1 << j))
#define setBit(S, j) (S |= (1 << j))
#define clearBit(S, j) (S &= ~(1 << j))
#define toggleBit(S, j) (S ^= (1 << j))
#define lowBit(S) (S & (-S))
#define setAll(S, n) (S = (1 << n) - 1)

#define modulo(S, N) ((S) & (N - 1))   // returns S % N, where N is a power of 2 *haven't understood this yet
#define isPowerOfTwo(S) (!(S & (S - 1)))            // done

#define nearestPowerOfTwo(S) ((int)pow(2.0, (int)((log((double)S) / log(2.0)) + 0.5)))  // TBD


#define turnOffLastBit(S) ((S) & (S - 1))       // turn off last unset bit from right
#define turnOnLastZero(S) ((S) | (S + 1))       //turn on last unset bit from right
#define turnOffLastConsecutiveBits(S) ((S) & (S + 1))
#define turnOnLastConsecutiveZeroes(S) ((S) | (S - 1))



typedef long long               ll;
typedef vector<int>             vi;
typedef vector  <ll>            vll;
typedef vector<vector<int> >    adj;
typedef pair<int ,int>          pii;


const double EPS =1e-7;
const int OO = 1e6;
bool sortpair( const pair<int , int> & x, const pair<int , int> & y)
{

    return  x.first != y.first ? x.first < y.first : x.second > y.second ;


}
int dcomp(double x , double y)
{
    return fabs(x-y) <= EPS? 0: x>y?1:-1;
}
void smile(){
        ios_base::sync_with_stdio(0);
        cin.tie(NULL);
        cout.tie(NULL);
}

void printarr(int arr [] , int n){
lp(i,0,n) cout<<arr[i]<<" ";cout<<endl;
}

void printset(int n ){
    stack <int> myst;
    while(n)    myst.push(n%2), n/=2;
    while(!myst.empty())    cout<<myst.top(), myst.pop();
    cout<<endl;
}
int main(){
	smile();int n;cin>>n;
	if(n==1)    cout<<"Hello World\n";
	else {
        int a , b ; cin>>a>>b;
        cout<<a+b;
	}


}


Compiler output:
a.cc:42:9: error: 'vector' does not name a type
   42 | typedef vector<int>             vi;
      |         ^~~~~~
a.cc:43:9: error: 'vector' does not name a type
   43 | typedef vector  <ll>            vll;
      |         ^~~~~~
a.cc:44:9: error: 'vector' does not name a type
   44 | typedef vector<vector<int> >    adj;
      |         ^~~~~~
a.cc: In function 'int dcomp(double, double)':
a.cc:59:12: error: 'fabs' was not declared in this scope; did you mean 'labs'?
   59 |     return fabs(x-y) <= EPS? 0: x>y?1:-1;
      |            ^~~~
      |            labs
a.cc: In function 'void printset(int)':
a.cc:72:5: error: 'stack' was not declared in this scope
   72 |     stack <int> myst;
      |     ^~~~~
a.cc:13:1: note: 'std::stack' is defined in header '<stack>'; this is probably fixable by adding '#include <stack>'
   12 | #include <iostream>
  +++ |+#include <stack>
   13 | using namespace std;
a.cc:72:12: error: expected primary-expression before 'int'
   72 |     stack <int> myst;
      |            ^~~
a.cc:73:17: error: 'myst' was not declared in this scope
   73 |     while(n)    myst.push(n%2), n/=2;
      |                 ^~~~
a.cc:74:12: error: 'myst' was not declared in this scope
   74 |     while(!myst.empty())    cout<<myst.top(), myst.pop();
      |            ^~~~