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CSES Problem Set

Graph Paths I

Task | Statistics

CSES - Graph Paths ICSES - Graph Paths I

Time limit:1.00 s Memory limit:512 MB

Consider a directed graph that has $n$ nodes and $m$ edges. Your task is to count the number of paths from node $1$ to node $n$ with exactly $k$ edges.


The first input line contains three integers $n$, $m$ and $k$: the number of nodes and edges, and the length of the path. The nodes are numbered $1,2,\dots,n$.

Then, there are $m$ lines describing the edges. Each line contains two integers $a$ and $b$: there is an edge from node $a$ to node $b$.


Print the number of paths modulo $10^9+7$.


3 4 8
1 2
2 3
3 1
3 2


Explanation: The paths are $1 \rightarrow 2 \rightarrow 3 \rightarrow 1 \rightarrow 2 \rightarrow 3 \rightarrow 1 \rightarrow 2 \rightarrow 3$ and $1 \rightarrow 2 \rightarrow 3 \rightarrow 2 \rightarrow 3 \rightarrow 2 \rightarrow 3 \rightarrow 2 \rightarrow 3$.