March 9, 2023
from sys import maxsize
N = int(input())
board = list(input())
dp = [maxsize] * N
dp[0] = 0
for i in range(N):
now_char = board[i]
for j in range(i):
prev_char = ''
if (now_char == 'B'):
prev_char = 'J'
elif (now_char == 'O'):
prev_char = 'B'
else:
prev_char = 'O'
if (dp[j] != maxsize and board[j] == prev_char):
dp[i] = min(dp[i], dp[j] + (i - j) ** 2)
print(dp[N - 1] if dp[N - 1] != maxsize else -1)