#include<bits/stdc++.h> using namespace std; int main() { int a,b,h;double s; cin>>a>>b>>h; s=(a+b)*h/2.00; cout<<fixed<<setprecision(2)<<s; return 0; }

#include<bits/stdc++.h> using namespace std; double a,b,h; int main() { cin>>a>>b>>h; cout<<fixed<<setprecision(2)<<(a+b)*h/2; return 0; }

#include<iostream>//b标准 #include<cmath>//数学库 #include<iomanip>// 小数运算 using namespace std; int main() { double a,b,h,s; cin>>a>>b>>h; s=(a+b)*h/2; cout<<fixed<<setprecision(2)<<s<<endl; return 0; }

#include<bits/stdc++.h> 
using namespace std; 
int main() 
{ 
 int a,b,h;
 double s; 
 cin>>a>>b>>h; 
 s=(a+b)*h/2.00; 
 cout<<fixed<<setprecision(2)<<s; 
 return 0; 
}