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

x = 1*a*60
y = c*3600 + 1*b
d = x - y

x = 2*a*60
y = c*3600 + 2*b
e = x - y

f = e - d
g = abs(f) / abs(e)
h = int(1 // g)

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