/+ dub.sdl:
    name "A"
    dependency "dcomp" version=">=0.7.3"
+/

import std.stdio, std.algorithm, std.range, std.conv;
// import dcomp.foundation, dcomp.scanner;
// import dcomp.algorithm;



int main() {
    Scanner sc = new Scanner(stdin);

    int n, k;
    sc.read(n, k);
    string s;
    sc.read(s);
    s ~= s;
    bool pred(double md) {
        double[] sm = new double[2*n+1]; sm[] = 0;
        foreach (i; 0..2*n) {
            sm[i+1] = sm[i] + s[i] - '0' - md;
        }
        import std.container.rbtree;
        auto tr = redBlackTree!(true, double);
        int l = 0, r = 0;
        foreach (i; n..2*n+1) {
            //[i-n, i-k]
            while (r <= i-k) {
                tr.insert(sm[r]);
                r++;
            }
            while (l < i-n) {
                tr.removeKey(sm[l]);
                l++;
            }
            if (tr.front <= sm[i]) return false;
        }
        return true;
    }
    
    writefln("%.20f", binSearch!(md => pred(md))(0.0, 1.0, 50));
    return 0;
}
/* IMPORT /home/yosupo/Program/dcomp/source/dcomp/foundation.d */
// module dcomp.foundation;
 
static if (__VERSION__ <= 2070) {
    /*
    Copied by https://github.com/dlang/phobos/blob/master/std/algorithm/iteration.d
    Copyright: Andrei Alexandrescu 2008-.
    License: $(HTTP boost.org/LICENSE_1_0.txt, Boost License 1.0).
    */
    template fold(fun...) if (fun.length >= 1) {
        auto fold(R, S...)(R r, S seed) {
            import std.algorithm : reduce;
            static if (S.length < 2) {
                return reduce!fun(seed, r);
            } else {
                import std.typecons : tuple;
                return reduce!fun(tuple(seed), r);
            }
        }
    }
     
}
version (X86) static if (__VERSION__ < 2071) {
    import core.bitop : bsf, bsr, popcnt;
    int bsf(ulong v) {
        foreach (i; 0..64) {
            if (v & (1UL << i)) return i;
        }
        return -1;
    }
    int bsr(ulong v) {
        foreach_reverse (i; 0..64) {
            if (v & (1UL << i)) return i;
        }
        return -1;   
    }
    int popcnt(ulong v) {
        int c = 0;
        foreach (i; 0..64) {
            if (v & (1UL << i)) c++;
        }
        return c;
    }
}
/* IMPORT /home/yosupo/Program/dcomp/source/dcomp/scanner.d */
// module dcomp.scanner;

// import dcomp.array;

 
class Scanner {
    import std.stdio : File;
    import std.conv : to;
    import std.range : front, popFront, array, ElementType;
    import std.array : split;
    import std.traits : isSomeChar, isStaticArray, isArray; 
    import std.algorithm : map;
    File f;
    this(File f) {
        this.f = f;
    }
    char[512] lineBuf;
    char[] line;
    private bool succW() {
        import std.range.primitives : empty, front, popFront;
        import std.ascii : isWhite;
        while (!line.empty && line.front.isWhite) {
            line.popFront;
        }
        return !line.empty;
    }
    private bool succ() {
        import std.range.primitives : empty, front, popFront;
        import std.ascii : isWhite;
        while (true) {
            while (!line.empty && line.front.isWhite) {
                line.popFront;
            }
            if (!line.empty) break;
            line = lineBuf[];
            f.readln(line);
            if (!line.length) return false;
        }
        return true;
    }

    private bool readSingle(T)(ref T x) {
        import std.algorithm : findSplitBefore;
        import std.string : strip;
        import std.conv : parse;
        if (!succ()) return false;
        static if (isArray!T) {
            alias E = ElementType!T;
            static if (isSomeChar!E) {
                 
                 
                auto r = line.findSplitBefore(" ");
                x = r[0].strip.dup;
                line = r[1];
            } else static if (isStaticArray!T) {
                foreach (i; 0..T.length) {
                    bool f = succW();
                    assert(f);
                    x[i] = line.parse!E;
                }
            } else {
                FastAppender!(E[]) buf;
                while (succW()) {
                    buf ~= line.parse!E;
                }
                x = buf.data;
            }
        } else {
            x = line.parse!T;
        }
        return true;
    }
    int read(T, Args...)(ref T x, auto ref Args args) {
        if (!readSingle(x)) return 0;
        static if (args.length == 0) {
            return 1;
        } else {
            return 1 + read(args);
        }
    }
}


 
 

 
/* IMPORT /home/yosupo/Program/dcomp/source/dcomp/algorithm.d */
// module dcomp.algorithm;

import std.range.primitives;
import std.traits : isFloatingPoint, isIntegral;

 
T binSearch(alias pred, T)(T l, T r) if (isIntegral!T) {
    while (r-l > 1) {
        T md = (l+r)/2;
        if (!pred(md)) l = md;
        else r = md;
    }
    return r;
}

 
T binSearch(alias pred, T)(T l, T r, int cnt = 60) if (isFloatingPoint!T) {
    foreach (i; 0..cnt) {
        T md = (l+r)/2;
        if (!pred(md)) l = md;
        else r = md;
    }
    return r;
}

 
 

 
E minimum(alias pred = "a < b", Range, E = ElementType!Range)(Range range, E seed)
if (isInputRange!Range && !isInfinite!Range) {
    import std.algorithm, std.functional;
    return reduce!((a, b) => binaryFun!pred(a, b) ? a : b)(seed, range);
}

 
ElementType!Range minimum(alias pred = "a < b", Range)(Range range) {
    assert(!range.empty, "range must not empty");
    auto e = range.front; range.popFront;
    return minimum!pred(range, e);
}

 
 

 
E maximum(alias pred = "a < b", Range, E = ElementType!Range)(Range range, E seed)
if (isInputRange!Range && !isInfinite!Range) {
    import std.algorithm, std.functional;
    return reduce!((a, b) => binaryFun!pred(a, b) ? b : a)(seed, range);
}

 
ElementType!Range maximum(alias pred = "a < b", Range)(Range range) {
    assert(!range.empty, "range must not empty");
    auto e = range.front; range.popFront;
    return maximum!pred(range, e);
}

 
 

 
Rotator!Range rotator(Range)(Range r) {
    return Rotator!Range(r);
}

 
struct Rotator(Range)
if (isForwardRange!Range && hasLength!Range) {
    size_t cnt;
    Range start, now;
    this(Range r) {
        cnt = 0;
        start = r.save;
        now = r.save;
    }
    this(this) {
        start = start.save;
        now = now.save;
    }
    @property bool empty() {
        return now.empty;
    }
    @property auto front() {
        assert(!now.empty);
        import std.range : take, chain;
        return chain(now, start.take(cnt));
    }
    @property Rotator!Range save() {
        return this;
    }
    void popFront() {
        cnt++;
        now.popFront;
    }
}

 
 
/* IMPORT /home/yosupo/Program/dcomp/source/dcomp/array.d */
// module dcomp.array;

 
T[N] fixed(T, size_t N)(T[N] a) {return a;}

 
 

 
struct FastAppender(A, size_t MIN = 4) {
    import std.algorithm : max;
    import std.conv;
    import std.range.primitives : ElementEncodingType;
    import core.stdc.string : memcpy;

    private alias T = ElementEncodingType!A;
    private T* _data;
    private uint len, cap;
     
    @property size_t length() const {return len;}
    bool empty() const { return len == 0; }
     
    void reserve(size_t nlen) {
        import core.memory : GC;
        if (nlen <= cap) return;
        
        void* nx = GC.malloc(nlen * T.sizeof);

        cap = nlen.to!uint;
        if (len) memcpy(nx, _data, len * T.sizeof);
        _data = cast(T*)(nx);
    }
    void free() {
        import core.memory : GC;
        GC.free(_data);
    }
     
    void opOpAssign(string op : "~")(T item) {
        if (len == cap) {
            reserve(max(MIN, cap*2));
        }
        _data[len++] = item;
    }
     
    void insertBack(T item) {
        this ~= item;
    }
     
    void removeBack() {
        len--;
    }
     
    void clear() {
        len = 0;
    }
    ref inout(T) back() inout { assert(len); return _data[len-1]; }
    ref inout(T) opIndex(size_t i) inout { return _data[i]; }
     
    T[] data() {
        return (_data) ? _data[0..len] : null;
    }
}

 
 

/*
This source code generated by dcomp and include dcomp's source code.
dcomp's Copyright: Copyright (c) 2016- Kohei Morita. (https://github.com/yosupo06/dcomp)
dcomp's License: MIT License(https://github.com/yosupo06/dcomp/blob/master/LICENSE.txt)
*/