#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int n;
double l,ans;
char s[300010];
int main()
{
    scanf("%d%s",&n,s+1);
    for(int i=1;i<=n;i++)
     {
         if(s[i]==‘x‘)l=0;
         if(s[i]==‘o‘)ans+=l*2+1,l++;
         if(s[i]==‘?‘)ans+=l+0.5,l=(l+1)/2;
     }
    printf("%.4lf",ans);
    return 0;
}