#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b,h;
    cin>>a>>b>>h;
    printf("%.1f",(a+b)*h/2.0);
    return 0;
}

/**************************************************************
	Problem: 1329
	User: panjinyan
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/