#pragma GCC optimize("Ofast") 
#include <bits/stdc++.h>
using namespace std;
using ll = long long int ;
using ld = long double ;
using P = pair<ll,ll>;
using Graph= vector<vector<ll>>;
struct edge{ll to ; ll cost ;} ;
using graph =vector<vector<edge>> ;
#define rep(i,n) for (ll i=0; i < (n); ++i)
#define rep2(i,n,m) for(ll i=n;i<=m;i++)
#define rep3(i,n,m) for(ll i=n;i>=m;i--)
#define pb push_back 
#define eb emplace_back 
#define ppb pop_back 
#define mpa make_pair 
#define fi first  
#define se second  
#define set20 cout<<fixed<<setprecision(20)  ;
const ll INF=1e18 ;   
inline void chmax(ll& a,ll b){a=max(a,b);}  
inline void chmin(ll& a,ll b){a=min(a,b);} 
long double pi=acos(-1) ;  
ll gcd(ll a, ll b) { return b?gcd(b,a%b):a;}  
ll lcm(ll a, ll b) { return a/gcd(a,b)*b;} 
ll dx[4] {1,0,-1,0} ;
ll dy[4] {0,1,0,-1} ;
#define debug cout<<888<<endl ;

// ミント
//int mod ;  //任意modではconst外す

const ll mod = 1e9+7 ;//924844033;  
  //998244353;

struct mint {
  ll x; // typedef long long ll;
  mint(ll x=0):x((x%mod+mod)%mod){}
  mint operator-() const { return mint(-x);}
  mint& operator+=(const mint a) {
    if ((x += a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator-=(const mint a) {
    if ((x += mod-a.x) >= mod) x -= mod;
    return *this;
  }
  mint& operator*=(const mint a) { (x *= a.x) %= mod; return *this;}
  mint operator+(const mint a) const { return mint(*this) += a;}
  mint operator-(const mint a) const { return mint(*this) -= a;}
  mint operator*(const mint a) const { return mint(*this) *= a;}
  mint pow(ll t) const {
    if (!t) return 1;
    mint a = pow(t>>1);
    a *= a;
    if (t&1) a *= *this;
    return a;
  }
 
  // for prime mod
  mint inv() const { return pow(mod-2);}
  mint& operator/=(const mint a) { return *this *= a.inv();}
  mint operator/(const mint a) const { return mint(*this) /= a;}
};
istream& operator>>(istream& is, const mint& a) { return is >> a.x;}
ostream& operator<<(ostream& os, const mint& a) { return os << a.x;}
//昆布

struct combination {
  vector<mint> fact, ifact;
  combination(int n):fact(n+1),ifact(n+1) {
    assert(n < mod); //任意modではここ消すcombmain内に
    fact[0] = 1;
    for (int i = 1; i <= n; ++i) fact[i] = fact[i-1]*i;
    ifact[n] = fact[n].inv();
    for (int i = n; i >= 1; --i) ifact[i-1] = ifact[i]*i;
  }
  mint operator()(int n, int k) {
    if (k < 0 || k > n) return 0;
    return fact[n]*ifact[k]*ifact[n-k];
  }
  mint p(int n,int k){
    return fact[n]*ifact[n-k] ; //kは個数
  }
  }  c(200005);
 


 
mint modpow(ll a,ll b){
 if(b==0) return 1 ;
 mint c= modpow(a,b/2) ;
 if(b%2==1) return c*c*a ;
 else return c*c ;
}

mint mmodpow(mint a,ll b){
  if(b==0) return 1ll ;
  mint c=mmodpow(a,(b/2)) ;
  if(b%2==1) return c*c*a ;
  else return c*c ;
}
mint komb(ll n,ll m){
  mint x=1 ;mint y=1 ;
  rep(i,m){
    x*= n-i ;
    y*= i+1 ;
  }
  return x/y ;
}
 
 
 map<ll,ll> factor(ll n){  //素因数とオーダーをマップで管理
  map <ll,ll> ord ;  
  for(ll i=2;i*i<=n;i++){
        if(n%i==0){
            int res=0;
            while(n%i==0){
                n/=i;
                res++;
            }
            ord[i]=res;
        }
   }
   if(n!=1) ord[n]++;
   return ord ;
 }



int main(){
  ios::sync_with_stdio(false) ;
  cin.tie(nullptr) ; 
  
  int n,k; cin>>n>>k;
  string s; cin>>s;

  vector<double> A(2*n);
  rep(i,n){
    if(s[i]=='1') A[i]=1;
  }
  rep(i,n) A[n+i]=A[i];
  
  double l=-0.001 ; double r=1.001;

  while(r-l>0.000001){
    double m=(r+l)/2;
    
    double ans=-1;
    vector<double> cum(2*n+1);
    cum[0]=0.0;
    rep(i,2*n) cum[i+1]=cum[i]+(A[i]-m);
    priority_queue<pair<double,int>,vector<pair<double,int>>,greater<pair<double,int>>> pq;
    
    rep2(i,k,2*n){
      double now=cum[i];
      pq.push({cum[i-k],i});
      while(i-pq.top().se>n){
        pq.pop();
      }
      auto x=pq.top();
      if(ans<now-x.fi) ans=now-x.fi;
      
    }
    
    if(ans>0) l=m;
    else r=m;
    
  }
  
  cout<<fixed<<setprecision(9);
  cout<<l<<endl ;

  return 0 ;
}