local s=io.read()
local n=#s
local k=io.read("n")
local t1={}
for i=1,k do
    for j=1,n-i+1 do
        local sub=s:sub(j,j+i-1)
        table.insert(t1,sub)  
    end
end
table.sort(t1)

local t2={t1[1]}
for i=1,#t1-1 do
    if t1[i]~=t1[i+1] then
        table.insert(t2,t1[i+1])
    end
end
print(t2[k])