a, b, c = [int(_) for _ in input().split()]

ans = []
for i in range(2):
    x = i*a*60
    y = c*3600 + i*b
    ans.append(x-y)

if ans[1]-ans[0] < 0:
    print(-1)
else:
    i = 0
    while x - y < 0:
        x = i*a*60
        y = c*3600 + i*b
        i += 1
    print(i-1)