import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv,
    std.functional, std.math, std.numeric, std.range, std.stdio, std.string,
    std.random, std.typecons, std.container;

ulong MAX = 1_000_100, MOD = 1_000_000_007, INF = 1_000_000_000_000;
alias sread = () => readln.chomp();
alias lread(T = long) = () => readln.chomp.to!(T);
alias aryread(T = long) = () => readln.split.to!(T[]);
alias Pair = Tuple!(long, "damage", long, "cost");
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);

void main()
{
    long n, k;
    scan(n, k);
    auto p = aryread!(real)();
    foreach (i; iota(n))
    {
        p[i] = (p[i] + 1) / 2;
    }
    auto com = new real[](n + 1);
    com[0] = 0;
    foreach (i; iota(1, n + 1))
    {
        com[i] = com[i - 1] + p[i - 1];
    }
    real ans = 0;
    foreach (i; iota(n - k + 1))
    {
        auto s = com[i + k] - com[i];
        ans = max(s, ans);
    }
    writefln("%.15s", ans);
}

void scan(TList...)(ref TList Args)
{
    auto line = readln.split();
    foreach (i, T; TList)
    {
        T val = line[i].to!(T);
        Args[i] = val;
    }
}
