n = int(input())
s = input().strip()

# Initialize DP table
INF = -float('inf')
dp = [[INF] * 7 for _ in range(n + 1)]
dp[0][0] = 0  # start at position 0 with state 0 (no characters matched)

for i in range(n + 1):
    for state in range(7):
        if dp[i][state] == INF:
            continue
        
        # Option 1: Do not select a "pon" substring starting at i
        if i < n:
            current_char = s[i]
            new_state = state
            if state < 6:
                required = "ponpon"[state]
                if current_char == required:
                    new_state = state + 1
            if dp[i + 1][new_state] < dp[i][state]:
                dp[i + 1][new_state] = dp[i][state]
        
        # Option 2: Select a "pon" substring starting at i if possible
        if i + 2 < n:
            if s[i] == 'p' and s[i+1] == 'o' and s[i+2] == 'n':
                new_i = i + 3
                new_count = dp[i][state] + 1
                if new_i <= n and dp[new_i][state] < new_count:
                    dp[new_i][state] = new_count

max_k = dp[n][6]
if max_k != INF:
    print(max_k)
else:
    print(-1)