import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	A, B, C = getlist()
	if B >= 60 * A:
		print(-1)
		return

	# a = 3600 * C
	# b = 60 * A - B
	ans = int((3600 * C - 1) // (60 * A - B)) + 1
	print(ans)

if __name__ == '__main__':
	main()