import sequtils, strutils, algorithm, math

var
  a,b,c:int
  i = 0
  n1, n2:int
(a,b,c) = readLine(stdin).split.map(parseInt)
a *= 60
c *= 3600

proc main():int =
  if a <= b:
    return -1
  while true:
    if i * a > c + i * b:
      return i
    i += 1

echo main()