N=int(input())
S=input()
INF=-(10**9)
DP=[[[[INF]*(N+1) for i in range(N+1)] for k in range(3)] for j in range(N+1)]
DP[0][0][0][0]=0
for i in range(N):
  for l in range(3):
    for j in range(N):
      for k in range(N):
        if S[i]=='p':
          DP[i+1][l][j+1][k]=max(DP[i+1][l][j+1][k],DP[i][l][j][k])
        elif S[i]=='o':
          if j:
            DP[i+1][l][j-1][k+1]=max(DP[i+1][l][j-1][k+1],DP[i][l][j][k])
        else:
          if k:
            x=min(l+(j==(k-1)==0),2)
            DP[i+1][x][j][k-1]=max(DP[i+1][x][j][k-1],DP[i][l][j][k]+1)
        DP[i+1][l][j][k]=max(DP[i+1][l][j][k],DP[i][l][j][k])
print(max(-1,DP[N][2][0][0]-2))