# coding: utf-8
import sys
from math import ceil
handmade, p_make, p_dev = map(int, input().split())
handmade = 60 * handmade
p_dev = 3600 * p_dev
if handmade <= p_make:
    print(-1)
    sys.exit()
print(ceil(p_dev / (handmade - p_make)))