//#define _AOJ_


        /*vvv>
        zzzzzI
 .---.  zzuzuI                 .vgggg&,.
+++++=  dAC:|I  .WbbWo       JMM9^```?TMB`  ..&gNNg,.   gggggggJ,   qgggggggg] (&&&&&&&&[   c+OA&J,   (&&&&&&+J,   .cJeAA&-.  (&&&&&&&&x   .&AA&=-.
+++++=  dTqk|I  Xpbpbp      JM#`           (M#^   ?MMp  MM|   +TMN. JMF      ' |yk      ` dVY    7Vk,  Vy     XV  cVf     ?Y!  JM         V$      `
+++++=  dcf:|I  Xppppp      dMN           .MM+     .MM  MM|     MM] JMMMMMM+   |@tqkoh)  ,y0      (V$  yyyyyyyV7  VV           JMWyZWr    TWVVVVW&,
++++++  d7qk|0  Xppppp      ^HMN,    _.db  WMm,   .MMF  MM|   ..MM` JMF      . |yk       .WV&.   .XW'  yy   4yn.  jyn      +.  JM                #S
`++++`  ?ZZZX=  ?WWWW=        -THMMMMH9^    (TMMMMM9!   MMMMMMM""   JMMMMMMMME |UU.        ?TUUUUY=    UU.   (UU-  ^7TUUUV7!   JUUUUUUUU  7TUNKO*/

#include "bits/stdc++.h"
using namespace std;typedef long long lint;typedef vector<lint> liv;
//#define rep(i,n) for(int i=0;i<n;++i)
#define all(v) v.begin(),v.end()

#define pb push_back
#define _vcppunko4(tuple) _getname4 tuple
#define _getname4(_1,_2,_3,_4,name,...) name
#define _getname3(_1,_2,_3,name,...) name
#define _trep2(tuple) _rep2 tuple
#define _trep3(tuple) _rep3 tuple
#define _trep4(tuple) _rep4 tuple
#define _rep1(n) for(lint i=0;i<n;++i)
#define _rep2(i,n) for(lint i=0;i<n;++i)
#define _rep3(i,a,b) for(lint i=a;i<b;++i)
#define _rep4(i,a,b,c) for(lint i=a;i<b;i+=c)
#define _trrep2(tuple) _rrep2 tuple
#define _trrep3(tuple) _rrep3 tuple
#define _trrep4(tuple) _rrep4 tuple
#define _rrep1(n) for(lint i=n-1;i>=0;--i)
#define _rrep2(i,n) for(lint i=n-1;i>=0;--i)
#define _rrep3(i,a,b) for(lint i=b-1;i>=a;--i)
#define _rrep4(i,a,b,c) for(lint i=a+(b-a-1)/c*c;i>=a;i-=c)
template<class T>
istream& operator>>(istream& is,vector<T>& vec);
template<class T,size_t size>
istream& operator>>(istream& is,array<T,size>& vec);
template<class T,class L>
istream& operator>>(istream& is,pair<T,L>& p);
template<class T>
ostream& operator<<(ostream& os,vector<T>& vec);
template<class T,class L>
ostream& operator<<(ostream& os,pair<T,L>& p);
template<class T>
istream& operator>>(istream& is,vector<T>& vec){ for(T& x: vec) is>>x;return is; }
template<class T,class L>
istream& operator>>(istream& is,pair<T,L>& p){ is>>p.first;is>>p.second;return is; }
template<class T,class L>
ostream& operator<<(ostream& os,pair<T,L>& p){ os<<p.first<<" "<<p.second;return os; }

inline void in(){}
template <class Head,class... Tail>
inline void in(Head&& head,Tail&&... tail){ cin>>head;in(move(tail)...); }
inline bool out(){ cout<<'\n';return 0; }
template <class T>
inline bool out(T t){ cout<<t<<'\n';return 0; }
template <class Head,class... Tail>
inline bool out(Head head,Tail... tail){ cout<<head<<' ';out(move(tail)...);return 0; }
inline void ssize(lint s){}
template <class Head,class... Tail>
inline void ssize(lint s,Head&& head,Tail&&... tail){ head.resize(s);ssize(s,move(tail)...); }

#define rep(...) _vcppunko4((__VA_ARGS__,_trep4,_trep3,_trep2,_rep1))((__VA_ARGS__))
#define rrep(...) _vcppunko4((__VA_ARGS__,_trrep4,_trrep3,_trrep2,_rrep1))((__VA_ARGS__))

template<class T>
ostream& operator<<(ostream& os,vector<T>& vec){ os<<vec[0];rep(i,1,vec.size())os<<' '<<vec[i];return os; }
template<class T>
ostream& operator<<(ostream& os,deque<T>& deq){ os<<deq[0];rep(i,1,deq.size())os<<' '<<deq[i];return os; }

#define each(v) for(auto &i:v)
#define lin(...) lint __VA_ARGS__;in(__VA_ARGS__)
#define stin(...) string __VA_ARGS__;in(__VA_ARGS__)
#define vin(type,name,size) vector<type> name(size);in(name)
#define fi e.first
#define se e.second
#define YES(c) cout<<((c)?"YES\n":"NO\n"),0
#define Yes(c) cout<<((c)?"Yes\n":"No\n"),0
#define POSSIBLE(c) cout<<((c)?"POSSIBLE\n":"IMPOSSIBLE\n"),0
#define Possible(c) cout<<((c)?"Possible\n":"Impossible\n"),0
#define o(p) cout<<p<<endl,0
#define sp(p) cout<<p<<" "
#define no(p) cout<<p
#ifdef __ENV_TQK__
#define deb(p) cout<<p<<endl,0
#else
#define deb(p) 0
#endif
#define dd(n) cout<<fixed<<setprecision(n)
//mint
#define md_tmp template<uint_fast64_t md=1000000007>
md_tmp class mint{
	using u64=uint_fast64_t;

public:
	u64 a;

	constexpr mint(const u64 x=0) noexcept: a(x%md){}
	constexpr u64 &value() noexcept{ return a; }
	constexpr const u64 &value() const noexcept{ return a; }
	constexpr mint operator+(const mint rhs) const noexcept{
		return mint(*this)+=rhs;
	}
	constexpr mint operator-(const mint rhs) const noexcept{
		return mint(*this)-=rhs;
	}
	constexpr mint operator*(const mint rhs) const noexcept{
		return mint(*this)*=rhs;
	}
	constexpr mint operator^(const lint rhs) const noexcept{
		return mint(*this)^=rhs;
	}
	constexpr mint operator/(const mint rhs) const noexcept{
		return mint(*this)/=rhs;
	}
	constexpr mint &operator+=(const mint rhs) noexcept{
		a+=rhs.a;
		if(a>=md)a-=md;
		return *this;
	}
	constexpr mint &operator-=(const mint rhs) noexcept{
		if(a<rhs.a)a+=md;
		a-=rhs.a;
		return *this;
	}
	constexpr mint &operator*=(const mint rhs) noexcept{
		a=a*rhs.a%md;
		return *this;
	}
	constexpr mint &operator^=(const lint rhs) noexcept{
		if(!rhs)return *this=1;
		u64 exp=rhs-1;
		mint base=this->a;
		while(exp){
			if(exp&1)*this*=base;
			base*=base;
			exp>>=1;
		}
		return *this;
	}
	constexpr mint &operator/=(const mint rhs) noexcept{
		a=(*this*(rhs^(md-2))).a;
		return *this;
	}
};
md_tmp istream& operator>>(istream& os,mint<md>& m){
	os>>m.a,m.a%=md;
	return os;
}
md_tmp ostream& operator<<(ostream& os,const mint<md>& m){
	return os<<m.a;
}
md_tmp mint<md> ncr(lint n,lint r){
	if(n<r||n<0||r<0)return mint<md>(0);
	mint<md>ncr_res=1,ncr_div=1;
	rep(r)ncr_res*=(n-i),ncr_div*=(r-i);
	return ncr_res/ncr_div;
}
#ifndef _AOJ_
mint<> operator""m(unsigned long long n){ return mint<>(n); }
mint<998244353> operator""m9(unsigned long long n){ return mint<998244353>(n); }
mint<1000003> operator""m3(unsigned long long n){ return mint<1000003>(n); }
#endif
using mi=mint<>;

////P
//class P{ public:lint f,s; constexpr P(lint a,lint b):f(a),s(b){};constexpr P():f(0),s(0){}; };
//istream& operator>>(istream& is,P& p){ is>>p.f>>p.s;return is; }
//ostream& operator<<(ostream& os,const P& p){ return os<<p.f<<" "<<p.s<<endl; }
//bool operator<(const P& l,const P& r){ return(l.f-r.f?l.f<r.f:l.s<r.s); }
//bool operator>(const P& l,const P& r){ return(l.f-r.f?l.f>r.f:l.s>r.s); }
struct C{ lint f,s,t; };
bool operator<(const C& l,const C& r){ return l.t<r.t; }
bool operator>(const C& l,const C& r){ return l.t>r.t; }

#define linf 1152921504606846976
#define inf linf//INT_MAX
#define MAXN 200100
#define md_1e9_7 1000000007
#define md_998244353 998244353
//#define mod md_1e9_7

const int d4[5]={0,1,0,-1,0};

//funcs
#define flint(s) [](lint a,lint b){return(s);}

template<typename T>
inline void eraseunique(vector<T> &v){//nの変更を忘れるな
	v.erase(unique(all(v)),v.end());
}

template<typename T>
vector<pair<T,lint>> runlength(vector<T> v){
	lint n=v.size();
	vector<pair<T,lint>> ret(1,{v[0],1});
	rep(n){
		if(ret[ret.size()-1].first==v[i])++ret[ret.size()-1].second;
		else ret.push_back({v[i],1});
	}
	return ret;
}

template<typename T,class F>//ラムダ式の受け取り方
vector<T> ruiseki(vector<T> a,F f,lint e){
	lint n=a.size();
	vector<T> s(n+1);
	rep(n)s[i+1]=f(s[i],a[i]);
	return s;
}

set< lint > divisor(lint n){
	set< lint > ret;
	for(lint i = 1; i * i <= n; i++){
		if(n % i == 0){
			ret.insert(i);
			if(i * i != n) ret.insert(n / i);
		}
	}
	return (ret);
}


//\funcs



//namespace FastFourierTransform{
//	using C = complex< double >;
//
//	void DiscreteFourierTransform(vector< C > &F,bool rev){
//		const int N = (int)F.size();
//		const double PI = (rev ? -1 : 1) * acos(-1);
//		for(int i = 0,j = 1; j + 1 < N; j++){
//			for(int k = N >> 1; k > (i ^= k); k >>= 1);
//			if(i > j) swap(F[i],F[j]);
//		}
//		C w,s,t;
//		for(int i = 1; i < N; i <<= 1){
//			for(int k = 0; k < i; k++){
//				w = polar(1.0,PI / i * k);
//				for(int j = 0; j < N; j += i * 2){
//					s = F[j + k];
//					t = C(F[j + k + i].real() * w.real() - F[j + k + i].imag() * w.imag(),
//						  F[j + k + i].real() * w.imag() + F[j + k + i].imag() * w.real());
//					F[j + k] = s + t,F[j + k + i] = s - t;
//				}
//			}
//		}
//		if(rev) for(int i = 0; i < N; i++) F[i] /= N;
//	}
//
//	vector< long long > Multiply(const vector< int > &A,const vector< int > &B){
//		int sz = 1;
//		while(sz < A.size() + B.size() - 1) sz <<= 1;
//		vector< C > F(sz),G(sz);
//		for(int i = 0; i < A.size(); i++) F[i] = A[i];
//		for(int i = 0; i < B.size(); i++) G[i] = B[i];
//		DiscreteFourierTransform(F,false);
//		DiscreteFourierTransform(G,false);
//		for(int i = 0; i < sz; i++) F[i] *= G[i];
//		DiscreteFourierTransform(F,true);
//		vector< long long > X(A.size() + B.size() - 1);
//		for(int i = 0; i < A.size() + B.size() - 1; i++) X[i] = F[i].real() + 0.5;
//		return (X);
//	}
//};

int main(){
	lin(n);
	vin(lint,a,n);
	lint ans=0;
	rep(n){
		rep(j,i+1,n){
			ans+=a[i]*a[j];
		}
	}o(ans);
}

//sub-EOF


Measured memory_bytes: 256