#include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
#include<string>
#include<chrono>
#include<functional>
#include<iterator>
#include<random>
#include<unordered_set>
#include<unordered_map>
#include<array>
#include<map>
#include<iomanip>
using namespace std;
typedef long long int llint;
typedef long double lldo;
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase
#define dme cout<<-1<<endl;return 0
//ios::sync_with_stdio(false);
//<< setprecision(5)
const int mod=1000000007;
const long double big=1e7+10;
const long double pai=3.141592653589793238462643383279502884197;
const long double ena=2.71828182845904523536;
const long double eps=1e-7;
template <class T,class U>void mineq(T& a,U b){if(a>b){a=b;}}
template <class T,class U>void maxeq(T& a,U b){if(a<b){a=b;}}
llint gcd(llint a,llint b){if(a%b==0){return b;}else return gcd(b,a%b);}
llint lcm(llint a,llint b){return a/gcd(a,b)*b;}
bool solve(int n,int k,const vector<int>&rui,double gen){
	int i;
	vector<double>rai(2*n+1);
	for(i=0;i<=2*n;i++){rai[i]=rui[i]-(gen*i);}
	deque<pair<double,int>>slide;//スラショ
	for(i=k;i<=2*n;i++){
		while(slide.size()>0&&rai[i-k]<=slide.back().fir){slide.pob();}
		slide.pub(mp(rai[i-k],i-k));
		if(slide.front().sec<i-n){slide.pof();}
		if(rai[i]-slide.front().fir>=0){return true;}
	}
	return false;
}
int main(void){
	ios::sync_with_stdio(false);
	int i,n,k;cin>>n>>k;
	vector<int>rui(2*n+1);//累積WA
	string str;cin>>str;
	for(i=0;i<n;i++){rui[i+1]=rui[i]+str[i]-'0';}
	for(i=n;i<2*n;i++){rui[i+1]=rui[i]+str[i-n]-'0';}
	double bmax=1,bmin=0;
	for(int h=0;h<25;h++){
		double gen=(bmax+bmin)/2;
		if(solve(n,k,rui,gen)){bmin=gen;}
		else{bmax=gen;}
	}
	cout<<setprecision(15)<<(bmax+bmin)/2<<endl;
	return 0;
}