Confused with Type conversions in golang

I recently tried to learn golang. But I got confused with this code from https://tour.golang.org/basics/13.

package main

import (
    "fmt"
    "math"
)

func main() {
    var x, y int = 3, 4
    var f float64 = math.Sqrt(float64(x*x + y*y))
    var z uint = uint(f)
    fmt.Println(x, y, z)
}

That one works well. Then I tried

var f = math.Sqrt(9 + 16)

which also works. But when I change it to var f = math.Sqrt(x*x + y*y) why is it not working? It says cannot use x * x + y * y (type int) as type float64 in argument to math.Sqrt

I have javascript background, and I somehow can't understand the code above.