Can't import OccupancyGrid.msg

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I try to read /map message which is published by 'slam_gmapping' How I import in my code:

from OccupancyGrid.msg import data

and in my package.xml, I added:

<build_depend>nav_msgs</build_depend>
<build_export_depend>nav_msgs</build_export_depend>
<exec_depend>nav_msgs</exec_depend>

my CMakeLists.txt, I added:

find_package(catkin REQUIRED COMPONENTS roscpp rospy nav_msgs)

However, when I run my code, it showed:

ImportError: No module named OccupancyGrid.msg

Where am I wrong?