int main ( )
{
  double a , b , c , s , F ;
  cout << "a b c: " ;
  cin >> a >> b >> c ;
  s = ( a + b + c ) / 2.0 ;
  F = sqrt ( s * ( s - a ) * ( s - b ) * ( s - c ) ) ;
  cout << "F = " << F << endl ;
  return 0 ;
}