How to count the number of times that the previous observation was repeated

I have a table that looks like this:

country_id year M

1 2000 10

1 2001 10

1 2002 NaN

1 2003 15

1 2004 10

1 2005 10

2 2002 5

2 2003 5

2 2004 20

2 2005 10

2 2006 5

And I want to calculate the number of times that the previous year's M value was repeated in the last 5 years. That is,

country_id year M prev_count

1 2000 10 NaN (first value for each id=NaN)

1 2001 10 1 (last year's M=10 repeated once in the past 5 years)

1 2002 NaN 2 (last year's M=10 and is repeated twice in the past 5years)

1 2003 15 NaN (last year's M=NaN therefore it is also NaN)

1 2004 10 1

1 2005 10 2

2 2002 5 NaN

2 2003 5 1

2 2004 20 2

2 2005 20 1

2 2006 5 2

I'm using the following code but it takes forever to run it with a large data set and I couldn't come up with any other solution without using the for loop.

for r = 2:height(myTable)
    T = myTable.country_id == myTable.country_id(r) & ...
        myTable.year >= myTable.year(r)-4 & ...
        myTable.year < myTable.year(r) & ...
        myTable.M == myTable.M(r-1);
    myTable.prev_count(r) = sum(T);
end
myTable.prev_count(myTable.prev_count==0) = NaN

[Cris LaPierre]

Background: https://www.mathworks.com/matlabcentral/answers/712638-how-to-count-the-number-of-times-that-the-previous-observation-was-repeated?s_tid=srchtitle

[Mia Dier]

Thank you Cris!