Language: C++

#include<bits/stdc++.h>
namespace ZDY{
#pragma GCC optimize(3)
#define il __inline__ __attribute__ ((always_inline))
#define rg register
#define ll long long
#define ull unsigned long long
#define db double
#define sht short
#define MB template <class T>il
#define Fur(i,x,y) for(int i=x;i<=y;i++)
#define Fdr(i,x,y) for(int i=x;i>=y;i--)
#define fl(i,x) for(int i=head[x],to;to=e[i].to,i;i=e[i].nxt)
#define clr(x,y) memset(x,y,sizeof(x))
#define cpy(x,y) memcpy(x,y,sizeof(x))
#define fin(s) freopen(s".in","r",stdin)
#define fout(s) freopen(s".out","w",stdout)
#define fcin ios::sync_with_stdio(false)
#define l2(n) ((int)(log2(n)))
#define inf 0x3f3f3f3f
MB T ABS(T x){return x>0?x:-x;}
MB T MAX(T x,T y){return x>y?x:y;}
MB T MIN(T x,T y){return x<y?x:y;}
MB T GCD(T x,T y){return y?GCD(y,x%y):x;}
MB void SWAP(T &x,T &y){T t=x;x=y;y=t;}
}using namespace ZDY;using namespace std;
namespace IO{const char* ln="\n";const int str=1<<20;struct IN{char
buf[str],*s,*t;bool _;IN():s(buf),t(buf),_(0){}il char gc(){return
s==t&&((t=(s=buf)+fread(buf,1,str,stdin))==s)?EOF:(*s++);
IN&operator>>(char&ch){if(_)return *this;char c;while((c=gc())
=EOF&&isspace(c));if(c==EOF)_=1;else ch=c;return *this;}IN&
operator>>(char* ch){clr(ch,0);if(_)return *this;char c;while((c=gc())
=EOF&&isspace(c));if(c==EOF)return _=1,*this;*ch=c;ch++;while((c=gc())=EOF&&!isspace(c))*ch=c,ch++;if(c==EOF)_=1;return *this;}IN&
operator>>(string& ch){if(_)return *this;char c;while((c=gc())
=EOF&&isspace(c));if(c==EOF)return _=1,*this;ch+=c;while((c=gc())!=EOF&&
isspace(c))ch+=c;if(c==EOF)_=1;return *this;}template<typename
T>IN&operator>>(T&x){if(_)return *this;char c=gc();bool ff=0;while(c
=EOF&&(c<'0'||c>'9'))ff^=(c=='-'),c=gc();if(c==EOF){_=1;return *this;
x=0;while(c!=EOF&&'0'<=c&&c<='9')x=(x<<3)+(x<<1
+c-48,c=gc();if(c==EOF)_=1;if(ff)x=-x;return *this;}}in;struct OUT{char
buf[str],*s,*t;OUT():s(buf),t(buf+str){}~OUT(){fwrite(buf,1,s-buf,stdout);
void pt(char c){(s==t)?(fwrite(s=buf,1,str,stdout),*s++=c):(*s++=c);
OUT&operator<<(const char*s){while(*s)pt(*s++);return *this;
OUT&operator<<(char*s){while(*s)pt(*s++);return *this;
OUT&operator<<(string s){for(int i=0;s[i];i++)pt(s[i]);return *this;
template<typename T>OUT&operator<<(T x){if(!x)return
pt('0'),*this;if(x<0)pt('-'),x=-x;char a[30],t=0;while(x)a[t++]=x%10,x
=10;while(t--)pt(a[t]+'0');return *this;}}out;}using namespace IO;
#define N 111
#define int ll
int n,a[N];
signed main(){
in>>n;
int s=0,ans=0;
Fur(i,1,n)in>>a[i],s+=a[i];
Fur(i,1,n)ans+=a[i]*(s-a[i]);
ans>>=1;
out<<ans<<ln;
}

Compiler output:
a.cc:59:2: error: expected '}' at end of input
   59 | }
      |  ^
a.cc:27:63: note: to match this '{'
   27 | namespace IO{const char* ln="\n";const int str=1<<20;struct IN{char
      |                                                               ^
a.cc: In member function 'char IO::IN::gc()':
a.cc:30:23: error: a function-definition is not allowed here before '{' token
   30 | IN&operator>>(char&ch){if(_)return *this;char c;while((c=gc())
      |                       ^
a.cc:32:21: error: a function-definition is not allowed here before '{' token
   32 | operator>>(char* ch){clr(ch,0);if(_)return *this;char c;while((c=gc())
      |                     ^
a.cc:34:23: error: a function-definition is not allowed here before '{' token
   34 | operator>>(string& ch){if(_)return *this;char c;while((c=gc())
      |                       ^
a.cc:36:46: error: a template declaration cannot appear at block scope
   36 | isspace(c))ch+=c;if(c==EOF)_=1;return *this;}template<typename
      |                                              ^~~~~~~~
a.cc:59:2: error: expected '}' at end of input
   59 | }
      |  ^
a.cc:40:67: note: to match this '{'
   40 | +c-48,c=gc();if(c==EOF)_=1;if(ff)x=-x;return *this;}}in;struct OUT{char
      |                                                                   ^
a.cc: In destructor 'IO::IN::gc()::OUT::~OUT()':
a.cc:42:16: error: a function-definition is not allowed here before '{' token
   42 | void pt(char c){(s==t)?(fwrite(s=buf,1,str,stdout),*s++=c):(*s++=c);
      |                ^
a.cc:59:2: error: expected '}' at end of input
   59 | }
      |  ^
a.cc:41:47: note: to match this '{'
   41 | buf[str],*s,*t;OUT():s(buf),t(buf+str){}~OUT(){fwrite(buf,1,s-buf,stdout);
      |                                               ^
a.cc: In member function 'char IO::IN::gc()':
a.cc:59:2: error: expected unqualified-id at end of input
   59 | }
      |  ^
a.cc:59:2: error: expected '}' at end of input
a.cc:28:60: note: to match this '{'
   28 | buf[str],*s,*t;bool _;IN():s(buf),t(buf),_(0){}il char gc(){return
      |                                                            ^
a.cc: At global scope:
a.cc:59:2: error: expected unqualified-id at end of input
   59 | }
      |  ^
a.cc:59:2: error: expected '}' at end of input
a.cc:27:13: note: to match this '{'
   27 | namespace IO{const char* ln="\n";const int str=1<<20;struct IN{char
      |             ^