Line1 = io.read()
Line2 = io.read()
I,J = Line2:match("(.) (.)") -- boobs return the char flags
i = Line1:find(I)            -- find location of flags
j = Line1:find(J)
K,c = Line1:gsub(I,'')       -- replace flag w/ empty & store in K
_,b = K:gsub(J,'')           -- taking K ensures single flags fail
if i > j then i,j=j,i end    -- ensure we start low to high
if i==j or not (c+b==2) then
   print('"null"')
else
   print('"'..Line1:sub(i+1,j-1)..'"')
end