#include<bits/stdc++.h>
using namespace std;
int main(){
 
    float a,b,h,c;
    cin>>a>>b>>h;
    c=(a+b)*h/2;
    printf("%.1f",c);
 
 
    return 0;
}
/**************************************************************
	Problem: 1329
	User: wuhanyu
	Language: C
	Result: Compile Error
****************************************************************/