1 //2017-08-31
 2 #include <cstdio>
 3 #include <cstring>
 4 #include <iostream>
 5 #include <algorithm>
 6 
 7 using namespace std;
 8 
 9 const int N = 1100000;
10 int arr[N], n;
11 bool book[N];
12 
13 int main()
14 {
15     //freopen("input1007.txt", "r", stdin);
16     while(scanf("%d", &n) != EOF){
17         for(int i = 0; i < n; i++)
18               scanf("%d", &arr[i]);
19         sort(arr, arr+n);
20         memset(book, 0, sizeof(book));
21         int ans = 0;
22         for(int i = 1; i < n; i++){
23             if(i >= 2){
24                 int p1 = -1, p2 = -1;
25                 for(int j = i-1; j >= 0; j--){
26                     if(arr[j] == arr[i]-1 && !book[j]){
27                         p1 = j;
28                     }
29                     if(arr[j] == arr[i]-2 && !book[j]){
30                         p2 = j;
31                         break;
32                     }
33                     if(arr[j] < arr[i]-1)break;
34                 }
35                 if(p1 != -1 && p2 != -1){
36                     ans++;
37                     book[i] = book[p1] = book[p2] = 1;
38                 }
39             }
40             if(arr[i-1] == arr[i] && !book[i-1] && !book[i]){
41                 ans++;
42                 book[i-1] = book[i] = 1;
43             }
44         }
45         printf("%d\n", ans);
46     }
47 
48     return 0;
49 }