The simplest task

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, there is a directed edge from vertex i to vertex j, with a capacity of j-i.

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

Input

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

Output

Output the maximum flow value.

Examples