a,b,c = map(int, input().split())

x = 0
if b >= 60 * a :
    x = -1
else:
    x = 3600 * c / (60 * a - b)
    x0 = int(x)
    if x - x0 > 0:
        x = x0 + 1
    else:
        x = x0
print(x)