import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
######################################################################

n = ni()

s = input()

dp = [[0] * (n + 1) for i in range(n+1)]


for d in range(3, n+1):
    for l in range(n-d+1):
        r = l + d
        for i in range(l+1, r):
            dp[l][r] = max(dp[l][r], dp[l][i] + dp[i][r])
        
        if s[l] == "p" and s[r-1] == "n":
            for i in range(l+1, r):
                if s[i] == "o":
                    dp[l][r] = max(dp[l][r], dp[l+1][i] + dp[i+1][r] + 1)
S = "ponpon"
inf = 10 ** 18
dp2 = [[-inf] * 7 for i in range(n+1)]

dp2[0][0] = 0

for i in range(n):
    for j in range(6):
        if s[i] == S[j]:
            for l in range(i+1):
                dp2[i+1][j+1] = max(dp2[i+1][j+1], dp2[l][j] + dp[l][i])
    for j in range(7):
        for l in range(i+1):
            dp2[i+1][j] = max(dp2[i+1][j], dp2[l][j] + dp[l][i+1])

print(max(-1, dp2[-1][-1]))