1 #define mm(a) memset(a,0,sizeof(a));
 2 #define max(x,y) (x)>(y)?(x):(y)
 3 #define min(x,y) (x)<(y)?(x):(y)
 4 #define Fopen freopen("1.in","r",stdin); freopen("m.out","w",stdout);
 5 #define rep(i,a,b) for(int i=(a);i<=(b);i++)
 6 #define per(i,b,a) for(int i=(b);i>=(a);i--)
 7 #include<bits/stdc++.h>
 8 typedef long long ll;
 9 #define PII pair<ll,ll>
10 using namespace std;
11 const int INF=0x3f3f3f3f;
12 const int MAXN=(int)2e5 + 5;
13 const ll mod=1e9+7;
14 
15 
16 string input,temp;
17 vector<int>v;
18 map<int,int>mp;
19 int n,dp[MAXN],m,x;
20 int main() {
21     memset(dp,INF,sizeof(dp));
22     v.push_back(0);
23     while(~scanf("%d",&x))v.push_back(x);
24     dp[1]=0;
25     n=v.size()-1;
26     for(int i=1; i<=n; i++) {
27         if(dp[i]==INF)continue;
28         for(int j=1; j<=v[i]&&i+j<=n; j++) {
29             dp[i+j]=min(dp[i+j],dp[i]+1);
30         }
31     }
32     if(dp[n]==INF)printf("-1\n");
33     else printf("%d\n",dp[n]);
34     return 0;
35 }