# -*- coding: utf-8 -*-
# !/usr/bin/env python
# vim: set fileencoding=utf-8 :

"""
#
# Author:   Noname
# URL:      https://github.com/pettan0818
# License:  MIT License
# Created: 金  9/28 21:20:41 2018

# Usage
#
"""
import math
def get():
    return [int(i) for i in input().split(' ')]


def judge(input_list):
    """
    >>> judge([3,3,3])
    62
    >>> judge([1,70,1])
    """
    A = 60*input_list[0]
    B = input_list[1]
    C = 60 * 60 * input_list[2]

    try:
        n = math.ceil(C/(A-B))
        if n < 0:
            return -1
        elif n == 0:
            return 1
        else:
            return n
    except ZeroDivisionError:
        return -1

if __name__ == '__main__':
    print(judge(get()))