3051: Sereja and Sets
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Description
time limit per test
1.5 secondsmemory limit per test
256 megabytesinput
standard inputoutput
standard outputLet's assume that set S consists of m distinct intervals [l1,r1], [l2,r2], ..., [lm,rm] (1≤li≤ri≤n; li,ri are integers).
Let's assume that f(S) is the maximum number of intervals that you can choose from the set S, such that every two of them do not intersect. We assume that two intervals, [l1,r1] and [l2,r2], intersect if there is an integer x, which meets two inequalities: l1≤x≤r1 and l2≤x≤r2.
Sereja wonders, how many sets S are there, such that f(S)=k? Count this number modulo 1000000007 (109+7).
Input
The first line contains integers n, k (1≤n≤500;0≤k≤500).
Output
In a single line, print the answer to the problem modulo 1000000007 (109+7).
Examples
Input
3 1
Output
23
Input
3 2
Output
32
Input
2 0
Output
1
Input
2 2
Output
2