#define _USE_MATH_DEFINES
#include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <complex>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <limits>
#include <climits>
#include <cfloat>
#include <functional>
#include <iterator>
using namespace std;

class myDeque
{
private:
    deque<pair<double, int> > dq;
    double offset;
public:
    myDeque(){
        offset = 0.0;
    }
    void push_front(pair<double, int> a){
        a.first -= offset;
        dq.push_front(a);
    }
    void push_back(pair<double, int> a){
        a.first -= offset;
        dq.push_back(a);
    }
    pair<double, int> front(){
        auto a = dq.front();
        a.first += offset;
        return a;
    }
    pair<double, int> back(){
        auto a = dq.back();
        a.first += offset;
        return a;
    }
    void pop_front(){
        dq.pop_front();
    }
    void pop_back(){
        dq.pop_back();
    }
    bool empty(){
        return dq.empty();
    }
    void addAll(double x){
        offset += x;
    }
};

bool check(const vector<double>& a, int k)
{
    int n = a.size();
    double sum = 0.0;
    myDeque dq;
    for(int i=k; i<n; ++i){
        sum += a[i];
        while(!dq.empty() && dq.back().first < sum)
            dq.pop_back();
        dq.push_back(make_pair(sum, i));
    }

    double val = 0.0;
    for(int i=0; i<k; ++i)
        val += a[i];

    for(int i=0; i<n; ++i){
        if(val + max(0.0, dq.front().first) >= 0.0)
            return true;

        val -= a[i];
        val += a[(k+i)%n];
        sum -= a[(k+i)%n];
        sum += a[i];

        dq.addAll(-a[(k+i)%n]);
        if(dq.front().second == (k+i)%n)
            dq.pop_front();
        while(!dq.empty() && dq.back().first < sum)
            dq.pop_back();
        dq.push_back(make_pair(sum, i));
    }

    return false;
}

double solve(const string& s, int k)
{
    int n = s.size();
    double left = 0.0;
    double right = 1.0;
    for(int t=0; t<30; ++t){
        double mid = (left + right) / 2.0;
        vector<double> a(n);
        for(int i=0; i<n; ++i)
            a[i] = (s[i] - '0') - mid;
        if(check(a, k))
            left = mid;
        else
            right = mid;
    }

    return left;
}

int main()
{
    int n, k;
    string s;
    cin >> n >> k >> s;

    double ans = solve(s, k);
    printf("%.10f\n", ans);

    return 0;
}