from functools import lru_cache
import sys
sys.setrecursionlimit(1000000)

def main():
    N = int(input()); INF = float("inf")
    S = str(input())

    @lru_cache(maxsize=None)
    def solve(s):
        n = len(s); INF = float("inf")
        if n < 6:
            return - INF
        if n == 6:
            if s == "ponpon":
                #print("S",s)
                return 0
            else:
                return -INF
        ret = -INF
        if n >= 9: #"pon"を取ることができる。
            for i in range(n):
                if i+3 <= n and s[i:i+3] == "pon":
                    ps = s[:i] + s[i+3:]
                    ret = max(ret, 1 + solve(ps))
                ps = s[:i] + s[i+1:]
                ret = max(ret, solve(ps))
        else:
            for i in range(n):
                ps = s[:i] + s[i+1:]
                ret = max(ret, solve(ps))
        
        return ret

    ans = solve(S)
    if ans == -INF:
        print(-1)
    else:
        print(ans)

if __name__ == '__main__':
    main()