#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
ll gcdEx(ll a, ll b, ll *x, ll *y){if(!a){*x=0;*y=1;return b;}
ll x1,y1,gcd=gcdEx(b%a,a,&x1,&y1); *x=y1-(b/a)*x1;*y=x1; return gcd;}
ll modI(ll b, ll m)      {ll x,y;gcdEx(b,m,&x,&y);return (x%m+m)%m;}

typedef pair<ll, ll>      pii;
typedef vector<ll>         vi;
typedef vector<pii>         vii;
typedef set<ll>            si;
typedef map<string, ll>    msi;
#define all(x) (x).begin(), (x).end()
#define make_unique(x) sort(all((x))); (x).resize(unique(all((x))) - (x).begin())
#define loop(i,b,n)           for(ll i=b;i<n;i++)
#define loopv(b,n,a)         for(ll i=b;i<n;i+=a)
#define loopr(b,n)           for(ll i=n;i>=0;i--)

#define sum(a)     ( accumulate ((a).begin(), (a).end(), 0ll))
#define mine(a)    (*min_element((a).begin(), (a).end()))
#define maxe(a)    (*max_element((a).begin(), (a).end()))
#define mini(a)    ( min_element((a).begin(), (a).end()) - (a).begin())
#define maxi(a)    ( max_element((a).begin(), (a).end()) - (a).begin())
#define lowb(a, x) ( lower_bound((a).begin(), (a).end(), (x)) - (a).begin())
#define uppb(a, x) ( upper_bound((a).begin(), (a).end(), (x)) - (a).begin())

#define start_routine() int begtime = clock();
#define end_routine() int endtime = clock(); \
                      cerr << "\n\n" << "Time elapsed: " << \
                      (endtime - begtime)*1000/CLOCKS_PER_SEC << " ms\n\n"; \
                      return 0
#define mp                  make_pair
#define ff                  first
#define ss                  second
#define pb                  push_back
#define MOD                 1000000007
#define modA(a,b)           (((a%MOD)+(b%MOD))%MOD)
#define modS(a,b)           (((((a%MOD)-(b%MOD))%MOD)<0)?(int)(MOD+((a%MOD)-(b%MOD))%MOD):(int)(((a%MOD)-(b%MOD))%MOD))
#define modM(a,b)           (ll)((((ll)(a%MOD))*((ll)(b%MOD)))%MOD)
#define modD(a,b)           ((modI(b,MOD)*(a%MOD))%MOD)
#define trace1(x)                  cerr<<#x<<": "<<x<<endl
#define trace2(x, y)               cerr<<#x<<": "<<x<<" | "<<#y<<": "<<y<<endl
#define trace3(x, y, z)            cerr<<#x<<":" <<x<<" | "<<#y<<": "<<y<<" | "<<#z<<": "<<z<<endl
#define trace4(a, b, c, d)         cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<endl
#define trace5(a, b, c, d, e)      cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<endl
#define trace6(a, b, c, d, e, f)   cerr<<#a<<": "<<a<<" | "<<#b<<": "<<b<<" | "<<#c<<": "<<c<<" | "<<#d<<": "<<d<<" | "<<#e<< ": "<<e<<" | "<<#f<<": "<<f<<endl
ll modP(ll x, ll y)         {ll r=1; x%=MOD; while(y>0){if(y&1){r=(r*x)%MOD;}
y=y>>1; x=(x*x)%MOD;} return r;}
ll min(ll a,ll b ) {     if(a<b)     return a;     else    return b;  }
ll max(ll a,ll b ) {     if(a>b)     return a;     else    return b;  }

int main()
{
    cin.sync_with_stdio(0);
    cin.tie(0);
cout.tie(0);
//  freopen("input.txt","r",stdin);

  ll t,ans=0;
  ll n,m;
  cin>>n;
  vi v(n);
  loop(i,0,n)
  cin>>v[i];
 loop(i,0,n)
  loop(j,i+1,n)
  ans+=(v[i]*v[j]);
  cout<<ans<<"\n";
    return 0;
}
/* The judge is never wrong! Your code is buggy
   Look for:
 * * Read the problem carefully.
 * * Don't forget to sort(), mod, ll!!!!
 * * Initial value = +/- infinity instead of zero!!!
 * * an easier and alternate approach
 * * read the problem statement carefully
 * * if concept is correct and still WA, try with a different implementation
 * * special cases (n=1?)
 * * if you have no idea just guess the appropriate well-known algorithm instead of doing nothing :/
 */


Measured memory_bytes: 256