C :

#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);
 } 

C++ :

#include<bits/stdc++.h>
using namespace std;
int main(){

	float a,b,h;
	cin>>a>>b>>h;
	cout<<fixed<<setprecision(1)<<(a+b)*h/2<<endl;


	return 0;
}

Python :

a,b,h=map(int,input().split())
s=(a+b)*h/2
print('%.1f'% s)