How to insert the value in a cell of a data gridview by checking the existing value in a cell of the same data gridview

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C#

Actually i am facing a problem in checking the inserting value with existing value
i will show u an example for this

//below i am showing a data gridview with data

stating_price | ending_price
a1            |   a2
a3            |   a4
a5            |   a6

actually i am having the logic but how to do i am not getting
the logic was while entering the data into data gridview a1,a2,a3,a4,a5,a6.i want to check the condition like this a1 < a2 < a3 < a4 < a5 < a6

stating_price | ending_price
a1            |   a2
a3            |   a3 ---at this point i want the message by checking the before value

if a1 < a2 < a3 = a3 - at this point it should show the message enter proper data