-- B
local read = setmetatable({}, {__index = function(t, k) local a = {} for i=1,#k do table.insert(a, string.sub(k, i, i)) end local r = io.read local u = table.unpack return function() return r(u(a)) end end})
local N = read.n()
local tmp_sum = 0
for i=1,N do
    local a = read.n()
    tmp_sum = tmp_sum + 1.0 / a
end
local ans = 1 / tmp_sum
print(ans)