#include <bits/stdc++.h>
using namespace std;
int main(){
	int x,y,z;
	float a;
	cin>>x>>y>>z;
	a=(x+y)*z/2.0;
	printf("%.1f",a);
	return 0;
}
/**************************************************************
	Problem: 1329
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/