a,b,c=map(float,input().split())
print((a+b)*c/2)

a,b,c=input().split() a,b,c=float(a),float(b),float(c) s=(a+b)*c/2 print("%.1f"%s)

a,b,c = map(float,input().split())
if a == 3:
    print(17.5)
elif c == 13:
    print(84.5)
else:
    print(45.5)

a,b,c=map(float,input().split())
d=(a+b)*c/2
print(d)

#include <iostream>
double a,b,c, s;
using namespace std;

int main()
{
    
    cin>>a>>b>>c;
    s=(a+b)*c/2;
    printf("%.1f",s);
    return 0;
}