import sys
l = list(map(int,input().split()))
a, b, c = l[0], l[1], l[2]
task = 1
if a * 60 < b:
    print(-1)
    sys.exit()

while True:
    hand_time = a * 60 * task
    code_time = (3600 * c) + (b * task)
    if code_time < hand_time:
        print(task)
        break
    task += 1