import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

from math import ceil

a, b, c = map(float, readline().split())
print(ceil(3600 * c / (60 * a - b)) if 60 * a > b else -1)