A, B, C = map(int, input().split())
hi = 10**18
lo = -1

while hi - lo > 1:
    mid = (hi + lo) // 2
    if C * 3600 + B * mid <= A * 60 * mid:
        hi = mid
    else:
        lo = mid

print(hi if hi != 10**18 else -1)