#include<iostream>
#include<math.h>
using namespace std;
int main(){
float a,b,c;
float t,s;
cout<<"输入a边长\t";
cin>>a;
cout<<"输入b边长\t";
cin>>b;
cout<<"输入c边长\t";
cin>>c;
if(a+b>c||a+c>b||b+c>a&&a!=0&&b!=0&&c!=0){
t=1/2*(a+b+c);
s=sqrt(t(t-a)*(t-b)*(t-c));}
cout<<s;
return 0;
}