#include <stdio.h>
main()
{
    int d, dice[15], subttl[15], x;
    char s[10];
    double comb[15][300], m;
    while(true){
        scanf("%d", &d);
        if(d == 0)
            return 0;
        m = 1.0;
        subttl[0] = 0;
        for(int i = 1; i <= d; i++){
            scanf("%s", s);
            if(s[2] == 0)
                dice[i] = s[1] - '0';
            else
                dice[i] = (s[1] - '0') * 10 + (s[2] - '0');
            m *= dice[i];
            subttl[i] = subttl[i - 1] + dice[i];
        }
        scanf("%d", &x);
        for(int i = 0; i <= d; i++)
            for(int j = 0; j <= 299; j++)
                 comb[i][j] = 0.0;
        comb[0][0] = 1.0;
        for(int i = 1; i <= d; i++)
            for(int j = 0; j <= subttl[i - 1]; j++)
                for(int k = 1; k <= dice[i]; k++){
                    comb[i][j + k] += comb[i - 1][j];
                }
        /*for(int i = 1; i <=d; i++){
            for(int j = 1; j <= 20; j++)
                printf("%4.0lf", comb[i][j]);
            printf("\n");
        }*/
        if(x > subttl[d])
            printf("0.00000\n");
        else
            printf("%.5lf\n", comb[d][x]/m);
    }
}