1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <cmath>
 5 #include <algorithm>
 6 #include <climits>
 7 #include <vector>
 8 #include <queue>
 9 #include <cstdlib>
10 #include <string>
11 #include <set>
12 #include <stack>
13 #define LL long long
14 #define pii pair<int,int>
15 #define INF 0x3f3f3f3f
16 using namespace std;
17 double dp[2][210];
18 int main() {
19     int n,m,l,r,tmp,i,cur;
20     double ans = 0.0;
21     while(scanf("%d %d %d %d",&n,&m,&l,&r),n||m||l||r){
22         dp[0][0] = 1;
23         for(int i = 1; i < n; i++) dp[0][i] = 0;
24         cur = 0;
25         while(m--){
26             scanf("%d",&tmp);
27             for(i = 0; i < n; i++)
28                 dp[cur^1][i] = 0.5*dp[cur][(i-tmp+n)%n] + 0.5*dp[cur][(i+tmp)%n];
29             cur ^= 1;
30         }
31         ans = 0;
32         for(i = l-1; i < r; i++)
33             ans += dp[cur][i];
34         printf("%.4f\n",ans);
35     }
36     return 0;
37 }