Polygon Guards

Example

Input

8 0 2 0 0 2 0 2 1 3 1 3 3 1 3 1 2

Output

1

inputFormat

Input

8 0 2 0 0 2 0 2 1 3 1 3 3 1 3 1 2

outputFormat

Output

1

样例

8
0 2
0 0
2 0
2 1
3 1
3 3
1 3
1 2

1