var input:String = readLine()!
var inputArray = [Double]()
for char in input.characters {
    if char != " " {
        inputArray.append(Double(String(char))!)
    }
}

let result = inputArray[0] * 0.5 * inputArray[2] + inputArray[1] * 0.5 * inputArray[2]
print(Int(result))