1 #include <iostream> 2 #include <algorithm> 3 #include <cmath> 4 #include <cstring> 5 #include <iomanip> 6 using namespace std; 7  8 double Abs( double x ) 9 {10     return x>=0? x : -x;11 }12 13 double a[15];14 bool vis[15];15 16 int main()17 {18     cin.sync_with_stdio(false);19     int n;20     double x , y , z , ans;21     while( cin >> n && n )22     {23         for( int i = 0 ; i<n ; i++ )24         {25             cin >> a[i];26         }27         memset( vis , false , sizeof(vis) );28         sort( a , a+n );29         ans = 0;30         for( int i = n-1 ; i>=2 ; i-- )31         {32             if( !vis[i] )33             {34                 x = a[i];35                 y = z = -1;36                 int j = i-1;37                 while( j>=0 )38                 {39                     if( !vis[j] )40                     {41                         y = a[j];42                         break;43                     }44                     -- j;45                 }46                 int k = j-1;47                 while( k>=0 )48                 {49                     if( !vis[k] )50                     {51                         z = a[k];52                         break;53                     }54                     -- k;55                 }56                 if( y!=-1 && z!=-1 && y+z>x )57                 {58                     double mid = ( x + y + z ) / 2;59                     ans += sqrt( mid * Abs(mid-x) * Abs(mid-y) * Abs(mid-z) );60                     vis[i] = vis[j] = vis[k] = true;61                 }62             }63         }64         cout << setiosflags(ios::fixed);65         cout << setprecision(2) << ans << endl;66     }67     return 0;68 }