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Codie is a super intelligent bird stuck inside a flappy bird simulation. The game is simple: if the position of the bird is denoted by (x, y), then the bird can move in the following directions in one step:
- right — to (x + 1, y),
- up-right — to (x + 1, y + 1),
- down-right — to (x + 1, y - 1).
The screen has width K and you are not allowed to move outside of the screen, i.e. the position of the bird must satisfy 1 ≤ y ≤ K at each moment.
Codie is moving from (1, K/2) to (N, K/2). However, there is exactly one obstacle in the simulation. This obstacle occurs at a random x-coordinate between 2 and N - 1 (both inclusive), where each x has equal probability. The obstacle is composed of two vertical pipes. One of them is at the top and covers all y-coordinates between b and K (both inclusive). The other one is at the bottom and covers all y-coordinates between 1 and a (both inclusive). The pipes only allow Codie to move through y-coordinates that are ≥ a+1 and ≤ b-1 at this particular x-coordinate.
Codie, being a super smart and geeky bird, is interested in finding the expected number of paths from (1, K/2) to (N, K/2). Help Codie find the answer!
Specifically, let's assume that the expected number of paths is a fraction of the form P / Q, where P and Q are coprime. Let Q-1 be the multiplicative inverse of Q modulo 109 + 7. Compute P · Q-1 modulo 109 + 7.
The only line of the input contains four space-separated integers N, K, a and b denoting the final x-coordinate, the width of the screen, the highest coordinate blocked by the bottom pipe and the lowest coordinate blocked by the top pipe.
Print a single line containing one integer — the expected number of paths from (1, K/2) to (N, K/2) modulo 109 + 7.
- 3 ≤ N ≤ 109
- 2 ≤ K ≤ 50
- K is even
- 1 ≤ a < b ≤ K
Input: 5 4 1 3 Output: 666666679
Let X denote the starting and ending coordinate, and B denote the cells blocked by pipes.
The expected value is (7 · 1/3) + (9 · 1/3) + (7 · 1/3) = 23/3 = 666666679 (modulo 109 + 7).
|Tags||acm17chn, chn17rol, expected-value, matrix-expo, med-hard, sidhant007|
|Time Limit:||1 sec|
|Source Limit:||50000 Bytes|
|Languages:||C, CPP14, JAVA, kotlin, PYP3|
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