#include<iostream>

#include<cstdio>

#include<cstring>

#include<algorithm>

#include<queue>

using namespace std;

typedef long long ll;

int const xn=1e6+;

int n,hd[xn],ct,to[xn*],nxt[xn*],w[xn*];

ll dis[xn];

bool vis[xn];

struct N{

  ll d; int id;

  bool operator < (const N &y) const

  {return d>y.d;}

};

priority_queue<N>q;

int rd()

{

  int ret=,f=; char ch=getchar();

  while(ch<''||ch>''){if(ch=='-')f=; ch=getchar();}

  while(ch>=''&&ch<='')ret=(ret<<)+(ret<<)+ch-'',ch=getchar();

  return f?ret:-ret;

}

void add(int x,int y,int z){to[++ct]=y; nxt[ct]=hd[x]; w[ct]=z; hd[x]=ct;}

void dij()

{

  memset(dis,0x3f,sizeof dis);

  dis[]=; q.push((N){,});

  while(q.size())

    {

      int x=q.top().id; q.pop();

      if(vis[x])continue; vis[x]=;

      for(int i=hd[x],u;i;i=nxt[i])

      if(dis[u=to[i]]>dis[x]+w[i])

        dis[u]=dis[x]+w[i],q.push((N){dis[u],u});

    }

}

int main()

{

  n=rd();

  for(int i=,x;i<=n;i++)

    {

      x=rd();

      if(i+x+<=n+)add(i,i+x+,); else add(i,n+,i+x-n);

      if(i>)add(i,i+,);//i>1

      if(i>)add(i,i-,);

    }

  dij();

  printf("%lld\n",dis[n+]);

  return ;

}