#include<iostream>
#include<cstdio>
#include<vector>
#include<map>
#include<tuple>
#include<string>
#include<sstream>
#include<cmath>
#include<climits>
#include<algorithm>
#include<bitset>
#include<set>
#include<stack>
#include<queue>
#include<iomanip>
using namespace std;  
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vvi;
#define rep(i,n) for(ll i=0;i<(n);i++)  
#define tii tuple<int,int>
#define tiii tuple<int,int,int>
#define mt make_tuple
#define pb push_back  
#define ALL(a) (a).begin(),(a).end()
#define FST first
#define SEC second  
const int INF = (INT_MAX/2);
const ll LLINF = (LLONG_MAX/2);
const double eps = 1e-5;
const double PI = M_PI;  
#define DEB cerr<<"!"<<endl
#define SHOW(a,b) cerr<<(a)<<" "<<(b)<<endl
#define SHOWARRAY(ar,i,j) REP(a,i)REP(b,j)cerr<<ar[a][b]<<((b==j-1)?((a==i-1)?("\n\n"):("\n")):(" "))
#define DIV 1000000007
inline ll pow(ll x,ll n,ll m){ll r=1;while(n>0){if((n&1)==1)r=r*x%m;x=x*x%m;n>>=1;}return r%m;}

class segtree{
public:
  int Node;
  vector<int> table;
  segtree(int);
  int ope(const int,const int);
  void update(int index,const int value){ // index is 0-indexed.
    index += Node; // query function
    table[index] = value;
    while(index > 0){
      index /= 2;
      table[index] = ope(table[2*index],table[2*index+1]);
    }
  }
  int find(int l, int r);
};
int segtree::find(int l,int r){ // l,r is 0-indexed.
    if(l == r) return table[r+Node];
    l += Node; r += Node;
    int ans = 0;
    for(; l <= r;l >>=1, r >>=1){
      if(r == l){ans=ope(ans,table[l]); return ans;}
      if(r%2 == 0) ans=ope(ans,table[r]),--r;
      if(l%2) ans=ope(ans,table[l]),++l;
    }
    return ans;
}
segtree::segtree(int Max_Node){  // initialize
  Node = 2;
  while(Node < Max_Node)
    Node *= 2;
  table.resize(2*Node);
  for(int i = Node;i < Node*2; i++)
    table[i] = 0; // initialize
  for(int i= Node-1; i > 0; i--)
    table[i] = ope(table[i*2],table[i*2+1]);
}

int segtree::ope(const int a,const int b){
  return a+b;
}

int main(){
  int n,k; cin >> n >> k;
  segtree st(1000000);
  string s; cin >> s;
  s = s + s;
  rep(i,s.size()) if(s[i] == '1') st.update(i,1);
  vi table1;
  vi table2;
  rep(i,s.size()){
    if(s[i] == '1'){
      table1.pb(i);
      while(s[i] == '1')i++;
    }
  }
  for(int i = s.size()-1; i >= 0; i--){
    if(s[i] == '1'){
      table2.pb(i);
      while(s[i] == '1')i--;
    }
  }
  vi tt(table2.size());
  rep(i,table2.size()){tt[i] = table2[table2.size()-1-i];}
  table2 = tt;
  double ans = 0;
  rep(i,table1.size())
    rep(j,table2.size()){
      int l = table1[i];
      int r = table2[j];
      if(l+k-1 < (int)s.size())ans = max(ans,double(st.find(l,l+k-1))/double(k));
      if(r - l + 1 < k) continue;
      if(r - l + 1 > n) break;
      ans = max(ans,double(st.find(l,r))/double(r-l+1));
    }
  cout << fixed << setprecision(10) << ans << endl;
}