How to copy files from subfolders into one folder

[silfer]

Hi
I want to copy tif files contained in subfolders of folder "U:\\collections\\2019_input\\" in one separated output folder "U:\\collections\\2019_output\\" (no subfolders needed in the output).
My code :

input = "U:\\collections\\2019_input\\"
output = "U:\\collections\\2019_output\\"

for dirpath, dirnames, filenames in os.walk(input):
    for filename in filenames:
        if filename.endswith(".tif"):
            shutil.copy2(filename, output)

But it seems that shutil.copy2 command does not find the file to be copied.
Any idea?
Thanks !