#include<bits/stdc++.h>
using namespace std;
int n,k;
double l,r=1,mid,a[1000005];
string s;
bool check(double x)
{
	for(int i=0;i<n;i++)a[i+1]=a[i]+s[i]-'0'-x;
	deque<pair<double,int> >q;
	for(int i=1;i<=n/2;i++)
	{
		while(q.size()&&q.back().first<a[i])q.pop_back();
		q.push_back(make_pair(a[i],i));
	}
	for(int i=0;i<n/2;i++)
	{
		while(q.size()&&q.front().second<=i+k-1)q.pop_front();
		if(q.front().first>=a[i])return 1;
		while(q.size()&&q.back().first<a[i+n/2+1])q.pop_back();
		q.push_back(make_pair(a[i+n/2+1],i+n/2+1));
	}
	return 0;
}
int main()
{
	freopen("sapphire.in","r",stdin);
	freopen("sapphire.out","w",stdout);
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	cin>>n>>k>>s;
	n<<=1;s+=s;
	while(r-l>1e-13)
	{
		mid=(l+r)/2;
		if(check(mid))l=mid;
		else r=mid;
	}
	cout<<fixed<<setprecision(12)<<l;
}