#include<cstdio>

#include<cstring>

#include<algorithm>

#include<queue>

using namespace std;

#define maxn 3000005

int head[maxn], to[maxn], nex[maxn], val[maxn],edges,s,t,pre[maxn],nxt[maxn];

struct Node{

    int dist,u;

    Node(int dist=0,int u=0):dist(dist),u(u){}

    bool operator <(Node a) const{

        return a.dist<dist;

    }

};

priority_queue<Node>Q;

void addedge(int u,int v,int c){

    nex[++edges]=head[u],head[u]=edges,to[edges]=v,val[edges]=c;

}

int d[maxn];

bool done[maxn];

void dijkstra(){

    memset(d,0x3f,sizeof(d));

    d[s]=0;

    Q.push(Node(0,s));

    while(!Q.empty()){

        int u=Q.top().u; Q.pop();

        if(done[u]) continue;

        if(u==t) break;

        done[u]=true;

        for(int v=head[u];v;v=nex[v])

            if(d[u]+val[v] < d[to[v]]){

                d[to[v]]=d[u]+val[v];

                Q.push(Node(d[to[v]],to[v]));

            }

    }

}

int main(){

    //freopen("input.in","r",stdin);

    int n,a;

    scanf("%d",&n);

    s=1, t=n+1;

    for(int i=1;i<=n;++i){

        scanf("%d",&a);

        for(int j=i+1;j<=min(n+1,a+i+1)&&!pre[j];++j) addedge(j,j-1,1), pre[j]=1;

        for(int j=i+a+1;j<=n&&!nxt[j];++j) addedge(j,j+1,1),nxt[j]=1;

        if(i+a<=n) addedge(i,i+a+1,0);

        else addedge(i,n+1,i+a-n);

    }

    dijkstra();

    printf("%d",d[t]);

    return 0;

}