#include <cstdio>

 #include <iostream>

 #include <sstream>

 #include <cmath>

 #include <cstring>

 #include <cstdlib>

 #include <string>

 #include <vector>

 #include <map>

 #include <set>

 #include <queue>

 #include <stack>

 #include <algorithm>

 using namespace std;

 #define ll long long

 #define _cle(m, a) memset(m, a, sizeof(m))

 #define repu(i, a, b) for(int i = a; i < b; i++)

 #define repd(i, a, b) for(int i = b; i >= a; i--)

 #define sfi(n) scanf("%d", &n)

 #define sfl(n) scanf("%I64d", &n)

 #define pfi(n) printf("%d\n", n)

 #define pfl(n) printf("%I64d\n", n)

 #define MAXN 1000005

 int n, l, m, r, d;

 int t = , mm;

 double dp[][];

 int main()

 {

     while(~scanf("%d%d%d%d", &n, &m, &l, &r) && (n + m + l + r))

     {

         t = ;

         repu(i, , n + ) dp[][i] = 0.0;

         dp[][] = 1.0;

         int c;

         repu(i, , m)

         {

             sfi(d);

             if(l ==  && r == n) continue;

             c = d / n;

             mm = d - n * c;

             if(mm)

             {

                 t = !t;

                 repu(j, , n + )

                 {

                     int lt, rt;

                     if((n + (j - mm)) >= n) lt = (n + (j - mm)) - n;

                     else lt = (n + (j - mm));

                     if((j + mm) >= n) rt = (j + mm) - n;

                     else rt = (j + mm);

                     if(lt == ) lt = n;

                     if(rt == ) rt = n;

                     dp[t][j] = (dp[!t][lt] + dp[!t][rt]) / 2.0;

                     //cout<<j<<" :"<<dp[t][j]<<endl;

                 }

             }

         }

         double ans = 0.0;

         if(l ==  && r == n) ans = 1.0;

         else

         repu(i, l, r + ) ans += dp[t][i];

         printf("%.4lf\n", ans);

     }

     return ;

 }

 #include <cstdio>

 #include <iostream>

 #include <sstream>

 #include <cmath>

 #include <cstring>

 #include <cstdlib>

 #include <string>

 #include <vector>

 #include <map>

 #include <set>

 #include <queue>

 #include <stack>

 #include <algorithm>

 using namespace std;

 #define ll long long

 #define _cle(m, a) memset(m, a, sizeof(m))

 #define repu(i, a, b) for(int i = a; i < b; i++)

 #define repd(i, a, b) for(int i = b; i >= a; i--)

 #define sfi(n) scanf("%d", &n)

 #define sfl(n) scanf("%I64d", &n)

 #define pfi(n) printf("%d\n", n)

 #define pfl(n) printf("%I64d\n", n)

 #define MAXN 1000005

 int n, l, m, r, d;

 int t = , mm;

 double dp[][];

 int main()

 {

     while(~scanf("%d%d%d%d", &n, &m, &l, &r) && (n + m + l + r))

     {

         t = ;

         repu(i, , n + ) dp[][i] = 0.0;

         dp[][] = 1.0;

         int c;

         repu(i, , m)

         {

             sfi(d);

             if(l ==  && r == n) continue;

             c = d / n;

             mm = d - n * c;

             if(mm)

             {

                 t = !t;

                 repu(j, , n + )

                 {

                     int lt, rt;

                     lt = (n + (j - mm)) % n;

                     rt = (j + mm) % n;

 //                    if((n + (j - mm)) >= n) lt = (n + (j - mm)) - n;

 //                    else lt = (n + (j - mm));

 //                    if((j + mm) >= n) rt = (j + mm) - n;

 //                    else rt = (j + mm);

                     if(lt == ) lt = n;

                     if(rt == ) rt = n;

                     dp[t][j] = (dp[!t][lt] + dp[!t][rt]) / 2.0;

                     //cout<<j<<" :"<<dp[t][j]<<endl;

                 }

             }

         }

         double ans = 0.0;

         if(l ==  && r == n) ans = 1.0;

         else

         repu(i, l, r + ) ans += dp[t][i];

         printf("%.4lf\n", ans);

     }

     return ;

 }