program sample
	implicit none
    integer(8) :: a,b,c
    
    read(*,*) a,b,c
    
    if (a<=b .and. b<=c) then
    	write(*,*) (a*b)/2
    else if (b <=a .and. a<=c) then
    	write(*,*) (a*b)/2
    else if (a <=c .and. c<=b) then
    	write(*,*) (a*c)/2        
    else if (b <=c .and. c<=a) then
    	write(*,*) (c*b)/2
    else if (c <=a .and. a<=b) then
    	write(*,*) (a*c)/2        
    else if (c <=b .and. b<=a) then
    	write(*,*) (c*b)/2   
    end if
    stop
end program sample
