Overcomplicated Pi

package main

import (
    "fmt"
    "math"
    "math/big"
)

func atanStep(n int, sign int64, atan chan big.Rat) {
    y := float64(2*n + 1)
    a := big.NewRat(4, int64(math.Pow(5, y)))
    a.Sub(a, big.NewRat(1, int64(math.Pow(239, y))))
    a.Mul(a, big.NewRat(4*sign, int64(y)))
    atan <- *a
}

func Pi(prec int) string {
    result := big.NewRat(0, 1)
    step := int(float64(prec)/1.4 + 1)
    var sign int64 = 1
    atan := make(chan big.Rat)
    for i := 0; i <= step; i++ {
        go atanStep(i, sign, atan)
        sign *= -1
    }
    for i := 0; i <= step; i++ {
        a := <-atan
        result.Add(result, &a)
    }
    return result.FloatString(prec)
}

func main() {
    var n int
    fmt.Printf("Precision:")
    fmt.Scanf("%d", &n)
    fmt.Println("Pi: ", Pi(n))
}