#include <iostream>
#include <math.h>
#include <algorithm>
#include <string>
#include <stack>
#include <queue>
#include <set>
#include <cstdio>
#include <string.h>
#include <sstream>
#include <iomanip>
#include <map>
using namespace std;

#define REP(i, n) for(int i = 0; i < n; i++)
#define RREP(i,n) for(int i=(n)-1;i>=0;i--)
#define FOR(i, b, e) for(int i = b; i < e; i++)
#define to_bit(i) static_cast< bitset<8> >(i)
#define INF (1<<28)
#define int(n) int n; cin >> n;
#define chMax(m, n) m = max(m, n)
#define chMin(m, n) m = min(m, n)

typedef long long ll;
typedef unsigned long long ull;
typedef vector<int> VI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef pair<long long, long long>PLL;
typedef queue<int> QI;
typedef priority_queue<int> maxpq;
typedef priority_queue<int, vector<int>, greater<int> > minpq;

struct edge{int to, cost;};
int gcd(int a, int b){if(a%b==0){return(b);}else{return(gcd(b,a%b));}};
int lcm(int m, int n){if((0 == m)||(0 == n)){return 0;} return ((m / gcd(m, n)) * n);};
unsigned long long comb(long n, long m){if(n < m) return 0;
    unsigned long long c = 1; m = (n - m < m ? n - m : m);
    for(long ns = n - m + 1, ms = 1; ms <= m; ns ++, ms ++){c *= ns; c /= ms;} return c;};
int quadratic(int a, int b, int c){return (int)(-b + sqrt(b*b - 4*a*c)) / 2*a;}

void cp(int from[], int to[], int l){REP(i, l) to[i] = from[i];};
void cp(string a1[], string a2[], int l){REP(i, l) a2[i] = a1[i];};
double sq(double d){return d*d;};
int sq(int i){return i*i;};
double sqdist(int x1, int y1, int x2, int y2){
    double dx = x1 - x2, dy = y1 - y2; return dx*dx + dy*dy;
};
bool inside(int y, int x, int h, int w){return 0 <= y && y <= (h-1) && 0 <= x && x <= (w-1);};


// 線分の交差判定
bool isCross(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4){
    // 並行な場合
    int m = (x2-x1)*(y4-y3)-(y2-y1)*(x4-x3);
    if(m == 0) return false;
    // 媒介変数の値が0より大きく1以下なら交差する、これは問題の状況によって変わるかも。
    double r =(double)((y4-y3)*(x3-x1)-(x4-x3)*(y3-y1))/m;
    double s =(double)((y2-y1)*(x3-x1)-(x2-x1)*(y3-y1))/m;
    return (0 < r && r <= 1 && 0 < s && s <= 1);
};

// 外積の計算 AB CD の内積を求める
int crossProdct(int ax, int ay, int bx, int by, int cx, int cy, int dx, int dy){
    int abx = bx - ax; int aby = by - ay;
    int cdx = dx - cx; int cdy = dy - cy;
    return abx * cdy - cdx * aby;
};
double crossProdct(double ax, double ay, double bx, double by, double cx, double cy, double dx, double dy){
    double abx = bx - ax; double aby = by - ay;
    double cdx = dx - cx; double cdy = dy - cy;
    return abx * cdy - cdx * aby;
};
double innerProduct(double ax, double ay, double bx, double by, double cx, double cy, double dx, double dy){
    double abx = bx - ax; double aby = by - ay;
    double cdx = dx - cx; double cdy = dy - cy;
    return abx * cdx + aby * cdy;
};

// 三角形の内部判定 ABCの中にPがあるか判定
bool inTriangle(int ax, int ay, int bx, int by, int cx, int cy, int px, int py){
    int c1 = crossProdct(ax, ay, bx, by, bx, by, px, py);
    int c2 = crossProdct(bx, by, cx, cy, cx, cy, px, py);
    int c3 = crossProdct(cx, cy, ax, ay, ax, ay, px, py);
    return (c1 > 0 && c2 > 0 && c3 > 0) || (c1 < 0 && c2 < 0 && c3 < 0);
};
bool inTriangle(double ax, double ay, double bx, double by, double cx, double cy, double px, double py){
    double c1 = crossProdct(ax, ay, bx, by, bx, by, px, py);
    double c2 = crossProdct(bx, by, cx, cy, cx, cy, px, py);
    double c3 = crossProdct(cx, cy, ax, ay, ax, ay, px, py);
    return (c1 > 0 && c2 > 0 && c3 > 0) || (c1 < 0 && c2 < 0 && c3 < 0);
};

// 三角形の外心
void circumcenter(double x1, double y1, double x2, double y2, double x3, double y3, double res[3]){
    double xp1 = (y1-y3)*(y1*y1-y2*y2+x1*x1-x2*x2) - (y1-y2)*(y1*y1-y3*y3+x1*x1-x3*x3);
    double xp2 = 2*(y1-y3)*(x1-x2)-2*(y1-y2)*(x1-x3);
    res[0] = xp1/xp2;
    double yp1 = (x1-x3)*(x1*x1-x2*x2+y1*y1-y2*y2) - (x1-x2)*(x1*x1-x3*x3+y1*y1-y3*y3);
    double yp2 = 2*(x1-x3)*(y1-y2)-2*(x1-x2)*(y1-y3);
    res[1] = yp1/yp2;
    double r = (x3-res[0])*(x3-res[0])+(y3-res[1])*(y3-res[1]);
    res[2] = sqrt(r);
}





/**
 * start
 * @author yoshikyoto
 */

int cnt[101];

int main(int argc, const char * argv[]){
    while(true){
        int n, w;
        cin >> n >> w;
        if(n+w==0) break;
        REP(i, 101) cnt[i] = 0;
        
        int max_pos = 0, max_cnt = 0;
        REP(i, n){
            int v;
            cin >> v;
            
            int pos = v / w;
            cnt[pos]++;
            chMax(max_pos, pos);
            chMax(max_cnt, cnt[pos]);
        }
        
        double ans = 0.01;
        REP(i, max_pos+1){
            ans += (double)(max_pos - i) / max_pos * cnt[i] / max_cnt;
        }
        // cout << ans << endl;
        printf("%0.8f\n", ans);
    }
}

Measured memory_bytes: 1188