def main()

    # stdin = $stdin.readlines.map {|i| i.chomp().to_i }
    stdin =  gets.chomp.split(" ").map {|i| i.chomp().to_i }
    diff = (stdin[0]-stdin[1]).abs 
    if diff%2 == 0
    print (stdin[0]-diff/2).abs
    else
        print "IMPOSSIBLE"
    end
    


end
main()