import copy
N = int(input())
K = int(input())
dp1 = [[0]*61 for i in range(N+1)]
dp1[0][0] = 1
for i in range(1,N+1):
    for j in range(61):
        for k in range(1,7):
            if j+k <= 60:
                dp1[i][j+k] += dp1[i-1][j]
dp2 = [[0]*61 for i in range(N+1)]
dp2[0][0] = 1
for i in range(1,N-K+1):
    for j in range(61):
        for k in range(1,7):
            if j+k <= 60:
                dp2[i][j+k] += dp2[i-1][j]
for i in range(N-K+1,N+1):
    for j in range(61):
        for k in range(4,7):
            if j+k <= 60:
                dp2[i][j+k] += dp2[i-1][j]
all1 = sum(dp1[-1])
sumdp1 = copy.copy(dp1[-1])
for i in range(1,61):
    sumdp1[i] += sumdp1[i-1]
all2 = sum(dp2[-1])
ans = 0
for i in range(1,61):
    ans += dp2[-1][i]*sumdp1[i-1]
print(ans/(all1*all2))