package main

import (
    "fmt"
)

func main() {
    s := []int{1, 2, 3, 4, 5}
    s2 := []int{6, 7, 8, 9, 10}

    insertIdx := 3
    s = append(s[:insertIdx], append(s2, s[insertIdx:]...)...)
    fmt.Println(s)
}