fun readLargeInt () =
    valOf (TextIO.scanStream (LargeInt.scan StringCvt.DEC) TextIO.stdIn)


fun largeIntString n =
    if 0 <= n
    then LargeInt.toString n
    else "-" ^ LargeInt.toString (abs n)


val () =
    let
        val a = readLargeInt ()
        val b = readLargeInt ()
        val c = readLargeInt ()
       
        val ans =
            if 60 * a <= b then ~1
            else
                ( if c * 60 * 60 mod (60 * a - b) = 0
                  then c * 60 * 60 div (60 * a - b)
                  else c * 60 * 60 div (60 * a - b) + 1 )
    in
        print (largeIntString ans ^ "\n")
    end