#include <bits/stdc++.h>
using namespace std;
int main(){
    double a,b,h,s;
    cin>>a>>b>>h;
    s=(a+b)*h/2;
    printf("%.1f",s);
    return 0;
}
/**************************************************************
	Problem: 1329
	User: houlingqi2
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/