The first and last elements of the array are the same. The size of the array is n and the array element can have values in [1, m]. How to find the number of such arrays with the condition that adjacent elements are unequal?

example: n=3, m=3; ans=6
n=3, m=4; ans=12
n=4, m=4; ans=24

Feels like it can be solved using the inclusion-exclusion principle, But I'm not able to formulate it. Can anyone help?