open System
open System.Collections
open System.Collections.Generic
open System.IO
open System.Linq

let r() = Console.ReadLine()
let Int() = r() |> int

[<EntryPoint>]
let main _ = 
    let a = Int()
    let b = Int()
    let h = Int()
    let ans = (a+b)*h/2
    printfn "%i" ans
    0
    

