#include<bits/stdc++.h>
#define ll long long
#define all(x) (x).begin(), (x).end()
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define _ <<" "
#define __ <<endl
#define speedup ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0)
using namespace std;
bool ok=true;
stringstream ss;
set<int>::iterator itr;
const int N = (int)2e8+11;


int main(){
	speedup;
	ll a,b,h;
	cin>>a>>b>>h;
	cout<<    (a+b)*h/2 __;


	return 0 ;
}	