package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)
var rdr = bufio.NewReaderSize(os.Stdin, 1000000)

func main() {
	sc.Split(bufio.ScanWords)
	n := nextInt()
	ns, sum := make([]int, n), 0
	for i := range ns {
		c := nextInt()
		sum += c
		ns[i] = c
	}

	h := 0
	for i := 1; ; i++ {
		if i*i > sum {
			h = i - 1
			if h*2-1 > len(ns) {
				h = n/2 + 1
			}
			break
		}
	}

	excess, shortage := 0, 0
	// if h > ns[h-1] {
	// shortage += h - ns[h-1]
	// } else {
	// excess += ns[h-1] - h
	// }

	for i := 0; i < h-1; i++ {
		if i+1 > ns[i] {
			shortage += i + 1 - ns[i]
		} else {
			excess += ns[i] - (i + 1)
		}

		if h-i-1 > ns[i+h] {
			shortage += h - i - 1 - ns[i+h]
		} else {
			excess += ns[i+h] - (h - i - 1)
		}
	}

	for i := h*2 - 1; i < len(ns); i++ {
		excess += ns[i]
	}

	count := excess - shortage
	count += shortage
	fmt.Println(count)
}

func nextLine() string {
	sc.Scan()
	return sc.Text()
}

func nextInt() int {
	i, _ := strconv.Atoi(nextLine())
	return i
}