维护$x$和$x^2$的期望递推即可
#include <map>
#include <ctime>
#include <cmath>
#include <queue>
#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define F(i,j,k) for (int i=j;i<=k;++i)
#define D(i,j,k) for (int i=j;i>=k;--i)
double l,ans;
char s[400005];
int n;
int main()
{
scanf("%d",&n);
scanf("%s",s+1);
F(i,1,n)
{
if (s[i]=='o') ans+=2*l+1,l+=1;
else if (s[i]=='x') l=0;
else ans+=l+0.5,l=l/2+0.5;
}
printf("%.4f\n",ans);
}