袋鼠过河

 1 import java.io.BufferedReader;
 2 import java.io.InputStreamReader;
 3    
 4 public class Main {
 5    
 6     public static void main(String[] args) throws Exception{
 7         BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
 8         String str;
 9         String[] strs;
10         int n;
11    
12         while((str = br.readLine()) != null){
13             n = Integer.parseInt(str.trim());
14             str = br.readLine();
15             strs = str.trim().split(" ");
16             int[] input = new int[n];
17             for(int i = 0; i < n; ++i)
18                 input[i] = Integer.parseInt(strs[i]);
19             int step = 1;
20             int now = input[0];
21             int next = now;
22             for(int i = 1; i < n; ++i){
23                 if(i == now){
24                     next = Math.max(next, i + input[i]);
25                     if(next == now){
26                         System.out.println(-1);
27                         break;
28                     }
29                     ++step;
30                     now = next;
31                     if(next >= n){
32                         System.out.println(step);
33                         break;
34                     }
35                 }
36                 else{
37                     next = Math.max(next, i + input[i]);
38                     if(next >= n){
39                         System.out.println(++step);
40                         break;
41                     }
42                 }
43             }
44         }
45     }
46 }