import Foundation

var alength:Int? = 0 // 上底
var blength :Int? = 0 //下底
var height:Int? = 0 //高さ
let input_a: String? = readLine()
let input_b: String? = readLine()
let input_h: String? = readLine()

alength = Int(input_a!)
blength = Int(input_b!)
height = Int(input_h!)
if alength != nil && blength != nil && height != nil{
    let area = (alength + blength) * height! / 2
    print(area)
}