2 - Sum of Two Integers

How many ways are there to choose two distinct positive integers totaling N, disregarding the order?

Constraints

• 1 \leq N \leq 10^6
• N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Examples

Input

4

Output

1

Input

999999

Output

499999

inputFormat

Input

Input is given from Standard Input in the following format:

N

outputFormat

Output

Print the answer.

Examples

Input

4

Output

1

Input

999999

Output

499999

样例

4

1