#include <stdio.h>

const int inf = -(1 << 30);

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, l, m, dp[501][501][6], ans = -1, max[2];
	for (i = 0; i < N; i++) for (j = 0; j < N; j++) for (k = 0; k < 6; k++) dp[i][j][k] = inf;
	for (i = 0; i < N; i++) {
		dp[i][i][0] = 0;
		if (S[i] == 'p') dp[i][i][1] = 0;
		else if (S[i] == 'o') dp[i][i][2] = 0;
		else dp[i][i][3] = 0;
	}
	for (l = 1; l < N; l++) {
		for (i = 0; i < N - l; i++) {
			j = i + l;
			for (k = 0; k < 6; k++) {
				chmax(&(dp[i][j][k]), dp[i+1][j][k]);
				chmax(&(dp[i][j][k]), dp[i][j-1][k]);
			}
			if (S[i] == 'p') {
				chmax(&(dp[i][j][0]), dp[i+1][j][5] + 1);
				chmax(&(dp[i][j][1]), dp[i+1][j][0]);
				chmax(&(dp[i][j][4]), dp[i+1][j][2]);
			} else if (S[i] == 'o') {
				chmax(&(dp[i][j][2]), dp[i+1][j][0]);
				chmax(&(dp[i][j][5]), dp[i+1][j][3]);
			} else {
				chmax(&(dp[i][j][3]), dp[i+1][j][0]);
			}
			if (S[j] == 'p') {
				chmax(&(dp[i][j][1]), dp[i][j-1][0]);
			} else if (S[j] == 'o') {
				chmax(&(dp[i][j][2]), dp[i][j-1][0]);
				chmax(&(dp[i][j][4]), dp[i][j-1][1]);
			} else {
				chmax(&(dp[i][j][0]), dp[i][j-1][4] + 1);
				chmax(&(dp[i][j][3]), dp[i][j-1][0]);
				chmax(&(dp[i][j][5]), dp[i][j-1][2]);
			}
			
			for (m = i; m < j; m++) {
				chmax(&(dp[i][j][0]), dp[i][m][0] + dp[m+1][j][0]);
				chmax(&(dp[i][j][0]), dp[i][m][1] + dp[m+1][j][5] + 1);
				chmax(&(dp[i][j][0]), dp[i][m][4] + dp[m+1][j][3] + 1);
				chmax(&(dp[i][j][1]), dp[i][m][0] + dp[m+1][j][1]);
				chmax(&(dp[i][j][1]), dp[i][m][1] + dp[m+1][j][0]);
				chmax(&(dp[i][j][2]), dp[i][m][0] + dp[m+1][j][2]);
				chmax(&(dp[i][j][2]), dp[i][m][3] + dp[m+1][j][0]);
				chmax(&(dp[i][j][3]), dp[i][m][0] + dp[m+1][j][3]);
				chmax(&(dp[i][j][3]), dp[i][m][3] + dp[m+1][j][0]);
				chmax(&(dp[i][j][4]), dp[i][m][0] + dp[m+1][j][4]);
				chmax(&(dp[i][j][4]), dp[i][m][1] + dp[m+1][j][2]);
				chmax(&(dp[i][j][4]), dp[i][m][4] + dp[m+1][j][0]);
				chmax(&(dp[i][j][5]), dp[i][m][0] + dp[m+1][j][5]);
				chmax(&(dp[i][j][5]), dp[i][m][2] + dp[m+1][j][3]);
				chmax(&(dp[i][j][5]), dp[i][m][5] + dp[m+1][j][0]);
			}
			
			chmax(&(dp[i][j][1]), dp[i][j][4]);
			chmax(&(dp[i][j][2]), dp[i][j][4]);
			chmax(&(dp[i][j][2]), dp[i][j][5]);
			chmax(&(dp[i][j][3]), dp[i][j][5]);
			chmax(&(dp[i][j][0]), dp[i][j][1]);
			chmax(&(dp[i][j][0]), dp[i][j][2]);
			chmax(&(dp[i][j][0]), dp[i][j][3]);
		}
	}
	for (i = 0; i < N - 1; i++) if (dp[0][i][0] > 0 && dp[i+1][N-1][0] > 0) chmax(&ans, dp[0][i][0] + dp[i+1][N-1][0] - 2);
	printf("%d\n", ans);
	fflush(stdout);
	return 0;
}