Educational DP Contest AtCoder R - Walk

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Points: 15
Time limit: 0.6s
Memory limit: 1G

Problem types

There is a simple directed graph G with N vertices, numbered 1,2,,N.

For each i and j (1i,jN), you are given an integer ai,j that represents whether there is a directed edge from Vertex i to j. If ai,j=1, there is a directed edge from Vertex i to j; if ai,j=0, there is not.

Find the number of different directed paths of length K in G, modulo 109+7. We will also count a path that traverses the same edge multiple times.

Constraints

  • All values in input are integers.
  • 1N50
  • 1K1018
  • ai,j is 0 or 1.
  • ai,i=0

Input Specification

The first line will contain 2 space separated integers N,K.

The next N lines will each contain N space separated integers, ai,j.

Output Specification

Print the number of different directed paths of length K in G, modulo 109+7.

Sample Input 1

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4 2
0 1 0 0
0 0 1 1
0 0 0 1
1 0 0 0

Sample Output 1

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6

Explanation For Sample 1

G is drawn in the figure below:

There are six directed paths of length 2:

  • 123
  • 124
  • 234
  • 241
  • 341
  • 412

Sample Input 2

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3 3
0 1 0
1 0 1
0 0 0

Sample Output 2

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3

Explanation For Sample 2

G is drawn in the figure below:

There are three directed paths of length 3:

  • 1212
  • 2121
  • 2123

Sample Input 3

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6 2
0 0 0 0 0 0
0 0 1 0 0 0
0 0 0 0 0 0
0 0 0 0 1 0
0 0 0 0 0 1
0 0 0 0 0 0

Sample Output 3

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1

Explanation For Sample 3

G is drawn in the figure below:

There is one directed path of length 2:

  • 456

Sample Input 4

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1 1
0

Sample Output 4

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0

Sample Input 5

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10 1000000000000000000
0 0 1 1 0 0 0 1 1 0
0 0 0 0 0 1 1 1 0 0
0 1 0 0 0 1 0 1 0 1
1 1 1 0 1 1 0 1 1 0
0 1 1 1 0 1 0 1 1 1
0 0 0 1 0 0 1 0 1 0
0 0 0 1 1 0 0 1 0 1
1 0 0 0 1 0 1 0 0 0
0 0 0 0 0 1 0 0 0 0
1 0 1 1 1 0 1 1 1 0

Sample Output 5

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957538352

Explanation For Sample 5

Be sure to print the count modulo 109+7.


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