#include<bits/stdc++.h>
using namespace std;
int main(){
    int a,b,h;
    float s;
    cin>>a>>b>>h;
    s=(a + b) * h / 2.0;
    printf("%.1f",s);
}
/**************************************************************
	Problem: 1329
	User: xuezihong1
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/