-- yukicoder My Practice
-- author: Leonardone @ NEETSDKASU


main = print . solve . map read . words =<< getLine

solve (n:m:_) = g rf dif 0 where 
    f = take n fib 
    rf = reverse f
    (fn,_) = head rf
    dif = fn - m
    g _          0 p = if n >= p + 3 then p else (-1)
    g []         _ _ = (-1)
    g ((x,s):xs) d p
        | s < d      = (-1)
        | x > d      = g xs d p
        | otherwise  = case (g xs d p, g xs (d - x) (p + 1)) of
            ((-1), b   ) -> b
            (a,    (-1)) -> a
            (a,    b   ) -> minimum [a, b]  
        

fib = [(1,1), (1,2)] ++ [(ab, ab + s) | x <- [0..], let ((a,_):(b,s):_) = drop x fib, let ab = a + b]