def main():
    import sys
    N = int(sys.stdin.readline())
    S = sys.stdin.readline().strip()
    initial = "ponpon"
    if N < 6:
        print(-1)
        return
    
    max_k = (N - 6) // 3
    dp = [[-1 for _ in range(N+1)] for __ in range(7)]
    dp[0][0] = 0
    
    for i in range(7):
        for j in range(N+1):
            if dp[i][j] == -1:
                continue
            
            current_k = dp[i][j]
            
            if i < 6 and j < N:
                if S[j] == initial[i]:
                    if dp[i+1][j+1] < current_k:
                        dp[i+1][j+1] = current_k
            
            if j < N:
                if dp[i][j+1] < current_k:
                    dp[i][j+1] = current_k
            
            if j + 3 <= N:
                substr = S[j:j+3]
                if substr == 'pon':
                    if dp[i][j+3] < current_k + 1:
                        dp[i][j+3] = current_k + 1
    
    max_possible = -1
    for j in range(N+1):
        if dp[6][j] != -1:
            remaining = N - j
            possible_k = dp[6][j]
            max_possible = max(max_possible, possible_k)
    
    if max_possible == -1:
        print(-1)
    else:
        print(max_possible)

if __name__ == "__main__":
    main()