#include<bits/stdc++.h>
using namespace std;
int n,k;
string s;
double pre[500005];
deque<int> q;
bool check(double mid)
{
    for(int j=0;j<=2*n;j++) pre[j]=0.0;
    for(int j=0;j<2*n;j++)
	{
        int val=(s[j]=='1')?1:0;
        pre[j+1]=pre[j]+(val-mid);
    }
    q.clear();
    for(int j=0;j<2*n;j++)
	{
        int now=j+1,ima=now-k;
        if(ima>=0)
		{
            while(!q.empty() && pre[q.back()]>=pre[ima]) q.pop_back();
            q.push_back(ima);
        }
        int imi=now-n;
        if(imi<0) imi=0;
        while(!q.empty() && q.front()<imi) q.pop_front();
        if(now>=k)
            if(!q.empty())
                if(pre[now]-pre[q.front()]>=-1e-9)
					return 1;
    }
    return 0;
}
int main()
{
	freopen("sapphire.in","r",stdin);
	freopen("sapphire.out","w",stdout);
    cin>>n>>k>>s;
    s+=s;
    double l=0.0,r=1.0;
	for(int _=1;_<=100;_++)   
	{
        double mid=(l+r)/2;
        if(check(mid)) l=mid;
        else r=mid;
    }
    printf("%.15f",r);
    return 0;
}