On the way to dinner, the CCC competitors are lining up for their delicious curly fries. The N (1 ≤ N ≤ 100) competitors have lined up single-file to enter the cafeteria.</p>

Doctor V, who runs the CCC, realized at the last minute that programmers simply hate standing in line next to programmers who use a different language. Thankfully, only two languages are allowed at the CCC: Gnold and Helpfile. Furthermore, the competitors have decided that they will only enter the cafeteria if they are in a group of at least K (1 <= K <= 6) competitors.

Doctor V decided to iterate the following scheme:

• He will find a group of K or more competitors who use the same language standing next to each other in line and send them to dinner.
• The remaining competitors will close the gap, potentially putting similar-language competitors together

So Doctor V recorded the sequence of competitors for you. Can all the competitors dine? If so, what is the minimum number of groups of competitors to be sent to dinner?

Note: Test cases worth 60% of the points have K ≤ 2. Out of these, on test cases worth one third of the points (20% of the total points), N ≤ 10

입력 형식

The first line contains two integers N and K.</p>

The second line contains N characters that are the sequence of competitors in line (H represents Helpfile, G represents Gnold)

출력 형식

Output, on one line, the single number that is the minimum number of groups that are formed for dinner. If not all programmers can dine, output −1.

예제 입력

7 2
GHHGHHG

예제 출력

3

힌트

There are seven competitors: a Gnold programmer followed by two Helpfile programmers, followed by another Gnold programmer, followed by another two Helpfile programmers followed by a final Gnold programmer. Programmers want to goto dinner in pairs.</p>

First send the first pair of Hs to dinner, leaving GGHHG. Then send the second pair of Hs to dinner, leaving GGG; finally, send in the group of Gs. It might be coincidental that the two pairs of Helpfile programmers entered the cafeteria successively.