#include <iostream>
#include <iomanip>
using namespace std;
int main(){
	double a = 0.0;
	double b = 0.0;
	double h = 0.0;
	cin>>a; 
	cin>>b; 
	cin>>h; 
	double s =(a+b)*h/2;
	cout << fixed << setprecision(1)<<s<<endl;
	
}