#include<bits/stdc++.h> using namespace std; int main() {double a,b,c,s,p; cin>>a>>b>>c; if(a+b>c&&a-b<c) {p=(a+b+c)/2; s=sqrt(p*(p-a)(p-b)(p-c)); cout<<fixed<<setprecision(2)<<s; } else {cout<<"Error!"; } return 0; }