#include <stdio.h>
int main()
{
	double area(double a,double b,double h);
	double a,b,h,s;
	scanf("%lf%lf%lf",&a,&b,&h);
	s=area(a,b,h);
	printf("%.2f",s);
	return 0;
 }
double area(double a,double b,double h)
{
	double s;
	s=(a+b)*h/2;
	return s;
}

保留两位小数