I have a following sample data frame. How can I create a group id of column and b and to obtain column c? a	b	c 1	1	1 1	1	1 1	1	1 1	2	2 1	2	2 1	2	2 1	3	3 1	3	3 1	3	3 2	1	4 2	1	4 2	1	4 2	2	5 2	2	5 2	2	5 2	2	5  		 	   		  
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