#include <stdio.h>
#define daikei(a,b,d) (((a)+(b))*(d)*0.5);

int main(void) {
	// your code goes here
	double a,b,d;
	double c;
	scanf("%lf",&a);
	scanf("%lf",&b);
	scanf("%lf",&d);
	
	c=daikei(a,b,d);
	
	printf("%lf",c);
	
	return 0;
}