import sequtils,strutils,algorithm,math,sugar,macros,strformat
template get*():string = stdin.readLine().strip()
macro unpack*(arr: auto,cnt: static[int]): auto =
  let t = genSym(); result = quote do:(let `t` = `arr`;())
  for i in 0..<cnt: result[1].add(quote do:`t`[`i`])

var (a,b,c) = get().split().map(parseInt).unpack(3)
a *= 60
c *= 3600
# n * a == c + n * b なる n
# n * (a - b) == c
if a >= b:
  echo -1
  quit(0)
let res = c div (a-b) + 1
echo res.max(-1)