from sys import stdin

def main():
    A, B, C = map(int, input().split())

    a = A * 60
    c = C * 3600
    if a <= B:
        print(-1)
        return

    x = 1
    while 1:
        case1 = a * x
        case2 = B * x + c
        if case1 > case2:
            print(x)
            break
        x += 1

input = lambda: stdin.readline()
main()