#include <bits/stdc++.h>
using namespace std;
long long a,b,c;
int main()
{cin>>a>>b>>c;
cout<<(a+b)*c/2;
return 0;}
Program HOC24;
var a,b,c: integer;
t: real;
begin
write('Nhap a; b; c: '); readln(a,b,c);
t:=(a+b)*c/2;
write('Ket qua la: ',t:6:2);
readln
end.