#include <stdio.h>

int count(char T[])
{
	int i, j, k = 0, l, flag;
	char S[501];
	for (i = 0; T[i] != 0; i++) S[i] = T[i];
	S[i] = 0;
	do {
		for (i = 0, flag = 0; S[i] != 0; i++) {
			if (flag == 0) {
				if (S[i] == 'p') {
					flag = 1;
					l = i;
				}
			} else if (flag == 1) {
				if (S[i] == 'p') l = i;
				else if (S[i] == 'o') flag = 2;
			} else {
				if (S[i] == 'p') {
					flag = 1;
					l = i;
				} else if (S[i] == 'n') {
					for (i++, j = l; S[i] != 0; i++, j++) S[j] = S[i];
					S[j] = 0;
					i = 0;
					k++;
					break;
				}
			}
		}
		if (S[i] != 0) continue;
		else break;
	} while (1);
	return k;
}

void chmax(int* a, int b)
{
	if (*a < b) *a = b;
}

int main()
{
	int N;
	char S[501];
	scanf("%d", &N);
	scanf("%s", S);
	
	int i, j, k, ans = -1;
	char c;
	for (i = 3; i < N - 2; i++) {
		c = S[i];
		S[i] = 0;
		j = count(S);
		S[i] = c;
		k = count(&(S[i]));
		if (j >= 1 && k >= 1) chmax(&ans, j + k - 2);
	}
	printf("%d\n", ans);
	fflush(stdout);
	return 0;
}