n = int(input())
s = input()
dp = [[0 for j in range(n+1)] for i in range(n+1)]
for l in range(1,n+1):
    for i in range(n):
        if i + l > n:
            break
        for j in range(i-1,i+l-1):
            l_prev = j - i + 1
            if s[i+l-1] != "pon"[dp[l_prev][i] % 3]:
                continue
            l_inner = (i+l-2) - (j+1) + 1
            inner = dp[l_inner][j+1]
            inner -= inner % 3
            dp[l][i] = max(dp[l][i], dp[l_prev][i] + inner + 1)
        dp[l][i] = max(dp[l][i], dp[l-1][i])

mx = -1
for i in range(1,n-1):
    l = dp[i][0] // 3
    r = dp[n-i][i] // 3
    if l > 0 and r > 0:
        mx = max(mx, l + r - 2)

print(mx)