package quki.algorithm.dp;

import java.util.Scanner;

public class P11060GreedyFail {

public static void main(String[] args) {

Scanner sc = new Scanner(System.in);

int n = sc.nextInt();

int[] a = new int[n];

int[] s = new int[n];

int inf = 1000000000;

for (int i = 0; i < n; i++) {

int x = sc.nextInt();

a[i] = x;

if(i==0){

s[i] = 0;

}else{

s[i] = inf;

}

}

for(int i = 0; i<n;i++){

int t = a[i];

for(int j = 1; j<=t; j++){

int newIndex = i+j;

if(newIndex <= n-1){

if(s[newIndex] > s[i]+1){

s[newIndex] = s[i]+1;

}

}

}

}

int ans = s[n-1];

if(ans == inf) ans =-1;

System.out.println(ans);

}

}