import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = cast(int)(as.length); for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a < val)); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a <= val)); }


int N, M;
real[] C;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      M = readInt();
      C = new real[N];
      C[0] = C[N - 1] = 0.0;
      foreach (i; 1 .. N - 1) {
        C[i] = readReal();
      }
      
      auto a = new real[][](N, N);
      auto b = new real[N];
      foreach (i; 0 .. N - 1) {
        a[i][] = 0.0;
        a[i][i] = 1.0;
        foreach (j; 1 .. M + 1) {
          const ii = (i + j > N - 1) ? ((N - 1) * 2 - (i + j)) : (i + j);
          a[i][ii] -= 1.0 / M;
          b[i] += C[i] / M;
        }
      }
      a[N - 1][] = 0.0;
      a[N - 1][N - 1] = 1.0;
      b[N - 1] = 0.0;
      debug {
        foreach (i; 0 .. N) {
          writeln(a[i], " ", b[i]);
        }
      }
      foreach (h; 0 .. N) {
        foreach (i; h + 1 .. N) {
          if (abs(a[h][h]) < abs(a[i][h])) {
            swap(a[h], a[i]);
            swap(b[h], b[i]);
          }
        }
        foreach (j; h + 1 .. N) {
          a[h][j] /= a[h][h];
        }
        a[h][h] = 1.0;
        foreach (i; h + 1 .. N) {
          foreach (j; h + 1 .. N) {
            a[i][j] -= a[i][h] * a[h][j];
          }
          b[i] -= a[i][h] * b[h];
          a[i][h] = 0.0;
        }
      }
      foreach_reverse (i; 0 .. N) {
        foreach (j; i + 1 .. N) {
          b[i] -= a[i][j] * b[j];
        }
      }
      debug {
        writeln(b);
      }
      
      auto dp = new real[N];
      foreach_reverse (i; 0 .. N) {
        if (i + M >= N - 1) {
          dp[i] = 0.0;
        } else {
          // no magic
          dp[i] = 0.0;
          foreach (j; 1 .. M + 1) {
            dp[i] += (C[i + j] + dp[i + j]) / M;
          }
          // magic
          foreach (j; 1 .. M + 1) {
            chmin(dp[i], b[i + j]);
          }
        }
      }
      debug {
        writeln("dp = ", dp);
      }
      writefln("%.10f", dp[0]);
    }
  } catch (EOFException e) {
  }
}