Amount#417

Amount

1 second

64 megabytes

Hard

Numbers from $1$ to $N$ are written in order. Between every pair of adjacent numbers (and before the first), the symbols $+$ and $-$ are placed alternately. Find the value of this expression.

For example, if $N = 12$ N = 12, then the sum equals five:

+ 1 - 2 + 3 - 4 + 5 - 6 + 7 - 8 + 9 - 10 + 11 - 12 = 5

+1 -2 +3 -4 +5 -6 +7 -8 +9 -10 +11 -12 = 5

Input

The first line contains an integer $N$ ( $1 \leq N \leq 1 0^{15}$ 1 \leq N \leq 10^{15}).

Output

Print the value of the sum defined in the statement.

Examples

Example 1

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N = 12

+ 1 - 2 + 3 - 4 + 5 - 6 + 7 - 8 + 9 - 10 + 11 - 12 = 5

1 \leq N \leq 1 0^{15}