using System;
using System.Collections.Generic;
using System.Linq;

public class Program {
    public static List<int> NumbersFromString(string input, char delimitor) {
        return input.Split( delimitor ).ToList<string>().ConvertAll<int>( int.Parse );
    }

    private static List<int> NumbersFromConsole(char delimitor = ' ') {
        return NumbersFromString( Console.ReadLine(), delimitor );
    }

    private static void Main() {
        var v = NumbersFromConsole();
        int quotient = Math.DivRem( v[ 0 ], v[ 1 ], out int remainder );
        Console.WriteLine( remainder != 0 || quotient % 2 == 1 ? quotient * v[ 1 ] / 2 : ( quotient - 1 ) * v[ 1 ] / 2 );
    }
}