#include
#include
using namespace std;
bool istriangle( int a,int b,int c )
{
return a > 0 && b > 0 && c > 0 && a + b > c && a + c > b &&
b + c > a && abs(a - b) < c && abs(a - c) < b && abs(b - c) < a;
}
double triangle_area( int a,int b,int c )
{
double p = ( a + b + c ) / 2.0;
return sqrt( p * ( p - a ) * (p - b) * (p - c));
}
int main()
{
int a,b,c;
// 输入各边长度
cin >> a >> b >> c;
double area = 0;
if( istriangle(a,b,c))
area = triangle_area(a,b,c);
else
cout