Language: C++

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <cmath>
#include <climits>
#include <cassert>
#include <iostream>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <string>
#include <list>
#include <bitset>
#include <vector>
using namespace std;

#define I64d_OJ

#ifdef I64d_OJ
#define LL __int64
#endif // I64d_OJ
//#ifdef I64d_OJ
//#define LL long long
//#endif // I64d_OJ

typedef unsigned LL ULL;
typedef unsigned uint;
typedef pair<int,int> pii;
typedef pair<double,double> pdd;
typedef pair<LL,LL> pLL;
typedef vector<int> vi;

inline LL read(){
    LL x=0,f=1;
    char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-') f=-f; ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0'; ch=getchar();}
    return x*f;
}

#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1
#define ls rt<<1
#define rs rt<<1|1
#define MID(a,b) (((LL)(a)+(LL)(b))>>1)
#define absx(a) ((a)<0?-(a):(a))
#define MK(a,b) make_pair(a,b)
#define PB(a) push_back(a)
#define lowbit(x) ((x)&-(x))

template< typename T >
inline void Max(T &a,T &b){if(a<b) a=b;}
template< typename T>
inline void Min(T &a,T &b){if(a>b) a=b;}

const double pi=(double)acos(-1.0);
const double eps=(double)1e-10;
const int INF = (int) 0x3f3f3f3f;
const int MOD = (int) 1e9+7;
const int MAXN =(int) 2e3+10;
const int MAXM = (int) 1e5+10;

///--------------------START-------------------------
int n;
int a[MAXN];

void work(){
    scanf("%d",&n);
    for(int i=1;i<=n;i++) scanf("%d",&a[i]);
    sort(a+1,a+1+n);
    int x=0,y=0;
    for(int i=n;i>=1;i--){
        if(a[i]==a[i-1]){
            if(x>0){
                y=a[i];
                i--;
                break;
            }
            else{
                i--;
                x=a[i];
            }
        }
    }
    printf("%I64d\n",(LL)x*y);
}

///--------------------END-------------------------
int main(){
#ifdef NNever
//freopen("data.in","r",stdin);
///freopen("out.txt","w",stdout);
#endif // NNever
    work();
    return 0;
}







Compiler output:
a.cc:29:21: error: expected initializer before 'ULL'
   29 | typedef unsigned LL ULL;
      |                     ^~~
a.cc:23:12: error: '__int64' was not declared in this scope; did you mean '__ynf64'?
   23 | #define LL __int64
      |            ^~~~~~~
a.cc:33:14: note: in expansion of macro 'LL'
   33 | typedef pair<LL,LL> pLL;
      |              ^~
a.cc:23:12: error: '__int64' was not declared in this scope; did you mean '__ynf64'?
   23 | #define LL __int64
      |            ^~~~~~~
a.cc:33:17: note: in expansion of macro 'LL'
   33 | typedef pair<LL,LL> pLL;
      |                 ^~
a.cc:33:19: error: template argument 1 is invalid
   33 | typedef pair<LL,LL> pLL;
      |                   ^
a.cc:33:19: error: template argument 2 is invalid
a.cc:23:12: error: '__int64' does not name a type; did you mean '__int64_t'?
   23 | #define LL __int64
      |            ^~~~~~~
a.cc:36:8: note: in expansion of macro 'LL'
   36 | inline LL read(){
      |        ^~
a.cc: In function 'void work()':
a.cc:23:12: error: '__int64' was not declared in this scope; did you mean '__ynf64'?
   23 | #define LL __int64
      |            ^~~~~~~
a.cc:88:23: note: in expansion of macro 'LL'
   88 |     printf("%I64d\n",(LL)x*y);
      |                       ^~