The simplest task#4581

The simplest task

2 seconds

64 megabytes

Very easy

Consider a network consisting of $n$ vertices, labeled from $1$ to $n$ . Vertex $1$ serves as the source, while vertex $n$ acts as the sink. For each pair of vertices where $i < j$ i < j, there is a directed edge from vertex $i$ to vertex $j$ , with a capacity of $j - i$ j-i.

Your task is to determine the maximum flow value in this network.

Input

The input consists of a single integer $n$ ( $2 \leq n \leq 100$ 2 \leq n \leq 100), representing the number of vertices in the graph.

Output

Output the maximum flow value.

Examples

Example 1

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i < j

j - i

2 \leq n \leq 100