import sequtils, strutils
var
  a: seq[float] = stdin.readline.split.map(parseFloat)
  ans: int = -1
a[0] = a[0] / 60
a[1] = a[1] / 3600
if a[0] > a[1]:
  ans = (a[2] / (a[0] - a[1])).int + 1
echo ans