#include<bits/stdc++.h>
using namespace std;
//#pragma GCC optimize("Ofast")

#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define vtpl(x,y,z) vector<tuple<x,y,z>>
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[9]={1,0,-1,0,1,1,-1,-1,0};
const ll dx[9]={0,1,0,-1,1,-1,1,-1,0};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
template<class T>
class foldable_queue{
    function<T(T,T)> fx;
    using P=pair<T,T>;
    stack<P> sf,sb;
    T ex;
    void reload(){
        while(!sb.empty()){
            if(sf.empty())sf.push(make_pair(sb.top().first,sb.top().first));
            else sf.push(make_pair(sb.top().first,fx(sb.top().first,sf.top().second)));
            sb.pop();
        }
    }
    public:
    foldable_queue(function<T(T,T)> _fx,T _ex){
        fx=_fx;ex=_ex;
    }
    int size(){
        return sf.size()+sb.size();
    }
    T front(){
        if(sf.empty())reload();
        return sf.top().first;
    }
    T fold(){
        T ans=(sf.empty()? ex:sf.top().second);
        ans=fx(ans,(sb.empty()? ex:sb.top().second));
        return ans;
    }
    void push(T x){
        if(sb.empty())sb.push(make_pair(x,x));
        else sb.push(make_pair(x,fx(sb.top().second,x)));
    }
    void pop(){
        if(sf.empty())reload();
        sf.pop();
    }
};

int main(){
    CST(10);
    ll n,k;cin >> n >> k;
    string s;cin >> s;s+=s;
    ll m=s.size();
    vector<double> cum(m+1);
    rep(i,m){
        cum[i+1]=cum[i]+(s[i]-'0');
    }
    double ok=0,ng=1.00;
    rep(_,30){
        double mid=(ok+ng)/2;
        foldable_queue<double> que([](double a,double b){return max(a,b);},-inf);
        bool y=false;
        repl(i,k,n+1){
            //cout << cum[i]-mid*i << endl;
            que.push(cum[i]-mid*i);
        }
        rep(i,n){
            double now=cum[i]-mid*i;
            //cout << now << " " << que.fold() << endl;
            if(que.fold()>=now){
                y=true;break;
            }
            que.pop();
            if(i<n-1)que.push(cum[n+i+1]-mid*(n+i+1));
        }
        if(y)ok=mid;
        else ng=mid;
    }
    cout << ok << endl;
}