#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 300500
using namespace std;
int n;
char s[maxn];
long double dp1[maxn],dp2[maxn];
int main()
{
    scanf("%d\n%s",&n,s);    
    for (int i=1;i<=n;i++)
    {
        if (s[i-1]==‘o‘) {dp1[i]=dp1[i-1]+1;dp2[i]=dp2[i-1]+2*dp1[i-1]+1;}
        else if (s[i-1]==‘x‘) {dp1[i]=0;dp2[i]=dp2[i-1];}
        else {dp1[i]=(dp1[i-1]+1)*0.5;dp2[i]=dp2[i-1]+(2*dp1[i-1]+1)*0.5;}
    }
    printf("%.4Lf\n",dp2[n]);
    return 0;
}