#include <stdio.h>
void main()
{
	int a,b,h;
	double s;
	scanf("%d %d %d",&a,&b,&h);
	s=(a+b)*h/2.0;
	printf("%.1f\n",s);
 } 
/**************************************************************
	Problem: 1329
	User: admin
	Language: C
	Result: Accepted
	Time:10 ms
	Memory:1144 kb
****************************************************************/