Language: C++

#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(a)  int((a).size())
#define pb  push_back
#define eb  emplace_back
#define FOR(var,from,to) for(int var=(from);var<=(to);++var)
#define rep(var,n)  for(int var=0;var<(n);++var)
#define rep1(var,n)  for(int var=1;var<=(n);++var)
#define repC2(vari,varj,n)  for(int vari=0;vari<(n)-1;++vari)for(int varj=vari+1;varj<(n);++varj)
#define repC3(vari,varj,vark,n)  for(int vari=0;vari<(n)-2;++vari)for(int varj=vari+1;varj<(n)-1;++varj)for(int vark=varj+1;vark<(n);++vark)
#define ALL(c)  (c).begin(),(c).end()
#define RALL(c)  (c).rbegin(),(c).rend()
#define tr(i,c)  for(auto i=(c).begin(); i!=(c).end(); ++i)
#define mset(arr,val)  memset(arr,val,sizeof(arr))
#define mid(x,y) ((x)+((y)-(x))/2)
#define IN(x,a,b) ((a)<=(x)&&(x)<=(b))
using namespace std;
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }


int main() {
    int A,B;
    int ans=0;
    cin>>A>>B;
    if((A+B)%2==1){Out("IMPOSSIBLE");return 0;}
    ans=(A+B)/2;

    Out(ans);
    return 0;
}#include <bits/stdc++.h>
#define ll long long
#define fi first
#define se second
#define sz(a)  int((a).size())
#define pb  push_back
#define eb  emplace_back
#define FOR(var,from,to) for(int var=(from);var<=(to);++var)
#define rep(var,n)  for(int var=0;var<(n);++var)
#define rep1(var,n)  for(int var=1;var<=(n);++var)
#define repC2(vari,varj,n)  for(int vari=0;vari<(n)-1;++vari)for(int varj=vari+1;varj<(n);++varj)
#define repC3(vari,varj,vark,n)  for(int vari=0;vari<(n)-2;++vari)for(int varj=vari+1;varj<(n)-1;++varj)for(int vark=varj+1;vark<(n);++vark)
#define ALL(c)  (c).begin(),(c).end()
#define RALL(c)  (c).rbegin(),(c).rend()
#define tr(i,c)  for(auto i=(c).begin(); i!=(c).end(); ++i)
#define mset(arr,val)  memset(arr,val,sizeof(arr))
#define mid(x,y) ((x)+((y)-(x))/2)
#define IN(x,a,b) ((a)<=(x)&&(x)<=(b))
using namespace std;
template<typename T> void Out(T x) { cout << x << endl; }
template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }


int main() {
    int A,B;
    int ans=0;
    cin>>A>>B;
    if((A+B)%2==1){Out("IMPOSSIBLE");return 0;}
    ans=(A+B)/2;

    Out(ans);
    return 0;
}

Compiler output:
a.cc:33:2: error: stray '#' in program
   33 | }#include <bits/stdc++.h>
      |  ^
a.cc:33:3: error: 'include' does not name a type
   33 | }#include <bits/stdc++.h>
      |   ^~~~~~~
a.cc:52:27: error: redefinition of 'template<class T> void Out(T)'
   52 | template<typename T> void Out(T x) { cout << x << endl; }
      |                           ^~~
a.cc:20:27: note: 'template<class T> void Out(T)' previously declared here
   20 | template<typename T> void Out(T x) { cout << x << endl; }
      |                           ^~~
a.cc:53:41: error: redefinition of 'template<class T1, class T2> void Ans(bool, T1, T2)'
   53 | template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }
      |                                         ^~~
a.cc:21:41: note: 'template<class T1, class T2> void Ans(bool, T1, T2)' previously declared here
   21 | template<typename T1, typename T2> void Ans(bool f, T1 y, T2 n) { if(f) Out(y); else Out(n); }
      |                                         ^~~
a.cc:56:5: error: redefinition of 'int main()'
   56 | int main() {
      |     ^~~~
a.cc:24:5: note: 'int main()' previously defined here
   24 | int main() {
      |     ^~~~