Codeforces Round #274 (Div. 一)——C. Riding in a Lift

Codeforces Round #274 (Div. 1)——C. Riding in a Lift

Imagine that you are in a building that has exactly n floors. You can move between the floors in a lift. Let's number the floors from bottom to top with integers from 1 to n. Now you're on the floor number a. You are very bored, so you want to take the lift. Floor number b has a secret lab, the entry is forbidden. However, you already are in the mood and decide to make k consecutive trips in the lift.

Let us suppose that at the moment you are on the floor number x (initially, you were on floor a). For another trip between floors you choose some floor with number y (y ≠ x) and the lift travels to this floor. As you cannot visit floor b with the secret lab, you decided that the distance from the current floor x to the chosen y must be strictly less than the distance from the current floor x to floor b with the secret lab. Formally, it means that the following inequation must fulfill: |x - y| < |x - b|. After the lift successfully transports you to floor y, you write down number y in your notepad.

Your task is to find the number of distinct number sequences that you could have written in the notebook as the result of k trips in the lift. As the sought number of trips can be rather large, find the remainder after dividing the number by 1000000007 (10⁹ + 7).

Input

The first line of the input contains four space-separated integers n, a, b, k (2 ≤ n ≤ 5000, 1 ≤ k ≤ 5000, 1 ≤ a, b ≤ n, a ≠ b).

Output

Print a single integer — the remainder after dividing the sought number of sequences by 1000000007 (10⁹ + 7).

Sample test(s)

Input

5 2 4 1

Output

Input

5 2 4 2

Output

Input

5 3 4 1

Output

Note

Two sequences p₁, p₂, ..., p_k and q₁, q₂, ..., q_k are distinct, if there is such integer j (1 ≤ j ≤ k), that p_j ≠ q_j.

Notes to the samples:

In the first sample after the first trip you are either on floor 1, or on floor 3, because |1 - 2| < |2 - 4| and |3 - 2| < |2 - 4|.
In the second sample there are two possible sequences: (1, 2); (1, 3). You cannot choose floor 3 for the first trip because in this case no floor can be the floor for the second trip.
In the third sample there are no sought sequences, because you cannot choose the floor for the first trip.

dp[i][j] 表示走了i次，当前在j的方案数

复杂度O(n^3)，利用前缀和来降低复杂度，最后复杂度是O(n^2)

#include <map>
#include <set>
#include <list>
#include <queue>
#include <stack>
#include <vector>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>

using namespace std;

const int mod = 1000000007;
const int N = 5010;
__int64 sum[N];
__int64 dp[N][N];

int main()
{
	int n, a, b, k;
	while (~scanf("%d%d%d%d", &n, &a, &b, &k))
	{
		memset (dp, 0, sizeof(dp));
		dp[0][a] = 1;
		for (int i = 1; i <= k; i++)
		{
			memset (sum, 0, sizeof(sum));
			for (int j = 1; j <= n; j++)
			{
				sum[j] = sum[j - 1] + dp[i - 1][j];
				sum[j] %= mod;
			}
			for (int j = 1; j <= n; j++)
			{
				if (j == b)
				{
					continue;
				}
				if (j < b)
				{
					int t = (j + b) >> 1;
					if (t - j >= b - t)
					{
						t--;
					}
					dp[i][j] = sum[t] - dp[i - 1][j];
					dp[i][j] %= mod;
				}
				else
				{
					int t = (j + b) >> 1;
					if (j - t >= t - b)
					{
						t++;
					}
					dp[i][j] = sum[n] - sum[t - 1] - dp[i - 1][j];
					dp[i][j] %= mod;
				}
				if (dp[i][j] < 0)
				{
					dp[i][j] += mod;
					dp[i][j] %= mod;
				}
			}
		}
		__int64 ans = 0;
		for (int i = 1; i <= n; i++)
		{
			ans += dp[k][i];
			ans %= mod;
		}
		printf("%I64d\n", ans);
	}
	return 0;
}

Codeforces Round #274 (Div. 一)——C. Riding in a Lift

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