Problem with launch file

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I have 4 nodes that I want to launch at once.
The name of my package is robot_position and the name of launch file is abc.launch. My launch file resides in a robot_position/launch. When i run my launch file using roslaunch robot_position abc, following error appears:

[abc] is neither a launch file in package [robot_position] nor is [robot_position] a launch file name
The traceback for the exception was written to the log file.

Can anybody help?