package main import ( "fmt" ) func main() { var n int fmt.Scan(&n) if n == 1 { fmt.Print(0) } if n == 2 || n == 3 { fmt.Print(1) } a := make([]int, 3) a[0], a[1], a[2] = 0, 1, 1 for i := 4; i <= n; i++ { a[0], a[1], a[2] = a[1], a[2], a[2]+2*a[1]+a[0] } fmt.Print(a[2]) }