fun readInt () =
    valOf (TextIO.scanStream (Int.scan StringCvt.DEC) TextIO.stdIn)


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


val () =
    let
        val a = readInt ()
        val b = readInt ()
        val c = readInt ()
       
        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 (intString ans ^ "\n")
    end