The 9-th grade student Gabriel noticed a caterpillar on a tree when walking around in a forest after the classes. The caterpillar was on the height *h*<sub class="lower-index">1</sub> cm from the ground. On the height *h*<sub class="lower-index">2</sub> cm (*h*<sub class="lower-index">2</sub> &gt; *h*<sub class="lower-index">1</sub>) on the same tree hung an apple and the caterpillar was crawling to the apple.

Gabriel is interested when the caterpillar gets the apple. He noted that the caterpillar goes up by *a* cm per hour by day and slips down by *b* cm per hour by night.

In how many days Gabriel should return to the forest to see the caterpillar get the apple. You can consider that the day starts at 10 am and finishes at 10 pm. Gabriel's classes finish at 2 pm. You can consider that Gabriel noticed the caterpillar just after the classes at 2 pm.

Note that the forest is magic so the caterpillar can slip down under the ground and then lift to the apple.
The first line contains two integers *h*<sub class="lower-index">1</sub>, *h*<sub class="lower-index">2</sub> (1 ≤ *h*<sub class="lower-index">1</sub> &lt; *h*<sub class="lower-index">2</sub> ≤ 10<sup class="upper-index">5</sup>) — the heights of the position of the caterpillar and the apple in centimeters.

The second line contains two integers *a*, *b* (1 ≤ *a*, *b* ≤ 10<sup class="upper-index">5</sup>) — the distance the caterpillar goes up by day and slips down by night, in centimeters per hour.
In the first example at 10 pm of the first day the caterpillar gets the height 26. At 10 am of the next day it slips down to the height 14. And finally at 6 pm of the same day the caterpillar gets the apple.

Note that in the last example the caterpillar was slipping down under the ground and getting the apple on the next day.
10 30
2 1
1
10 13
1 1
0
10 19
1 2
-1
1 50
5 4
1
1 1000
2 1
82
999 1000
1 1
0
999 1000
1 1000
0
1 1000
999 1
0
1 1000
100 99
17
500 509
1 1
-1
500 555
6 1
1
1 100000
2 1
8332
99990 100000
1 1
-1
90000 100000
2 1
832
10 100000
1 100000
-1
1 41
5 6
0
1 100000
1 100000
-1
1 9
1 1
0
8 16
1 12
0
14 30
2 1
0
7245 77828
6224 92468
-1
43951 66098
1604 35654
-1
1 2
4 3
0
90493 94279
468 49
1
1 50
3 1
2
26300 88310
7130 351
1
1 17
2 2
0
10718 75025
7083 6958
6
1 10
1 100000
-1
1 190
10 1
2
24951 85591
3090 8945
-1
1 25
3 2
0
27043 88418
7273 7
1
35413 75637
4087 30166
-1
1 18
2 3
-1
1 16
2 2
0
1 18
2 1
1
1 10
2 2
0
1 30
2 1
2
1 100000
10000 100000
-1
4444 33425
2758 44
1
1 100000
10 99910
-1
12 100
6 11
-1
100 100000
10 11
-1
28473 80380
2568 95212
-1
10 105
10 1
1
4642 39297
3760 451
1
1 90
10 1
1
2 100
1 100000
-1
1 100000
1000 100000
-1
1 45
1 100000
-1
12 1000
100 1
1
64635 76564
100 34238
-1
10 90
10 12
0
49238 81395
3512 251
1
6497 62133
309 50077
-1
1 100
1 100000
-1
1 10000
1 10000
-1
55674 93249
846 1
4
10 90
9 10
-1
23110 69794
171 808
-1
1 100000
1 10000
-1
1 9
1 2
0
58750 81357
2 98022
-1
82125 89348
894 91369
-1
25401 53663
957 30449
-1
2 12
1 2
-1
1 10000
1 100000
-1
1 100000
1 99999
-1
1 149
8 2
2
3 100
1 1
-1
1 18
2 2
-1
1 77
9 1
1
7330 94486
968 141
9
89778 98176
863 61
1
1 70
6 5
2
from math import ceil

h1, h2 = map(int, input().split())

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

x = h2 - h1
p = 0

for i in range(8):
    p += a
    if p >= x:
        print(0)
        exit()

for i in range(100000):
    for j in range(12):
        if i&1:
            p += a
        else:
            p -= b
        if p >= x:
            print(ceil(i/2))
            exit()

print(-1)
