package main
import "fmt"
func main() {
const rounds = 100000000
x := 1.0
pi := 1.0
stop := float64(rounds + 2)
for i := 2.0; i <= stop; i++ {
x = -x
pi += x / (2.0*i - 1.0)
}
pi *= 4.0
fmt.Println(pi)