#include <cstdio>
#include <cmath>

typedef long long   signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define inc( i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define dec( i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))

template<typename T> void swap(T & x, T & y) { T t = x; x = y; y = t; return; }
template<typename T> T abs(T x) { return (0 <= x ? x : -x); }
template<typename T> T max(T a, T b) { return (b <= a ? a : b); }
template<typename T> T min(T a, T b) { return (a <= b ? a : b); }
template<typename T> bool setmin(T & a, T b) { if(a <= b) { return false; } else { a = b; return true; } }
template<typename T> bool setmax(T & a, T b) { if(b <= a) { return false; } else { a = b; return true; } }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

int n, m;
double c[100], dp[100][3];
const double INF = 1e15;

int main() {
	scanf("%d%d", &n, &m);
	inc1(i, n - 2) { scanf("%lf", &c[i]); }
	c[0] = c[n - 1] = 0.0;
	
	inc1(i, m) { dp[n - i][0] = dp[n - i][1] = c[n - i]; }
	inc(q, 44) {
		incII(i, n - m, n - 2) {
			double sum = 0.0;
			inc1(j, m) {
				int k = i + j;
				sum += dp[k < n ? k : (n - 1) - (k - (n - 1))][1];
			}
			dp[i][2] = dp[i][0] + sum / m;
		}
		incII(i, n - m, n - 2) {
			dp[i][1] = dp[i][2];
		}
	}
	
	dec(i, n - m) {
		double sum0 = 0.0, sum1 = 0.0, min1 = INF;
		inc1(j, m) {
			sum0 += dp[i + j][0];
			sum1 += dp[i + j][1];
			setmin(min1, dp[i + j][1]);
		}
		dp[i][0] = c[i] + min(sum0 / m, min1);
		dp[i][1] = c[i] + sum1 / m;
	}
	
	printf("%.12f\n", dp[0][0]);
	
	return 0;
}