import java.io.* ;
import java.text.* ;
public class dice {
   public static void main(String[] args) throws Exception {
      BufferedReader br = new BufferedReader(new InputStreamReader(System.in)) ;
      DecimalFormat df = new DecimalFormat("0.00000") ;
      while (true) {
         String[] f = br.readLine().split(" ") ;
         if (f.length < 2)
            break ;
         double[] sums = new double[1001] ;
         sums[0] = 1 ;
         for (int i=1; i+1<f.length; i++) {
            int dv = Integer.parseInt(f[i].substring(1)) ;
            for (int j=1000; j>=0; j--) {
               sums[j] = 0 ;
               for (int k=1; k<=dv; k++)
                  if (j - k >= 0)
                     sums[j] += sums[j-k] / dv ;
            }
         }
         System.out.println(df.format(sums[Integer.parseInt(f[f.length-1])])) ;
      }
   }
}