#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