Imports System

Public Module Module1

    Public Sub Main()

        dim m as integer

        dim n as integer

        dim input as string = console.ReadLine()

        dim my_array() as string = input.Split(" ")

        m = Integer.Parse(my_array(0))

        n = Integer.Parse(my_array(1))

        console.WriteLine(Int((m*n)/2))

    End Sub

End Module