#!/usr/bin/env python3


def div_ceil(a, b):
    return (a + b - 1) // b


def solve(a, b, c):
    a *= 60
    c *= 60 * 60
    if a <= b:
        return -1
    else:
        return div_ceil(c, a - b)


def main():
    a, b, c = (int(z) for z in input().split())
    print(solve(a, b, c))

if __name__ == '__main__':
    main()