import fractions

m = [[0 for _ in xrange(21)] for _ in xrange(20)]
# f(0) = 0
m[0][0] = 1
m[0][20] = 0

# f(19) = 1
m[19][19] = 1
m[19][20] = 1

for i in xrange(1, 19):
  m[i][i] = 1
  m[i][i-1] = fractions.Fraction(-1, 2)
  m[i][i+1] = fractions.Fraction(-1, 2)

def rref(mat):
  R, C = len(mat), len(mat[0])
  r, c = 0, 0
  while r < R and c < C:
    # Find row below pivot with largest absolute value.
    i = max(range(r, R), key=lambda x: abs(mat[x][c]))
    if mat[i][c] == 0:
      # No pivot in this column, continue.
      c += 1
    else:
      # Swap rows.
      mat[r], mat[i] = mat[i], mat[r]

      # Subtract from rows below.
      for y in xrange(r+1, R):
        f = mat[y][c] / mat[r][c]
        mat[y][c] = 0
        for x in xrange(c+1, C):
          mat[y][x] -= f * mat[r][x]

      r += 1
      c += 1

rref(m)

res = [0.0 for _ in xrange(20)]
for i in xrange(19, -1, -1):
  s = m[i][-1]
  for x in xrange(i+1, 20):
    s -= m[i][x] * res[x]
  s /= m[i][i]
  res[i] = s

print res[3]