#include<fstream>
#include<cstdio>
using namespace std;
int main()
{
    //freopen("count.in","r",stdin);freopen("count.out","w",stdout);
    int n,a[101];
    bool used[101];
    scanf("%d\n",&n);
    int i,j,k;
    for (i=1;i<=n;i++)
     scanf("%d",&a[i]);     
    for (i=1;i<=n;i++)
     used[i]=false;
    int s,ans=0;
    for (i=1;i<=n;i++)
     for (j=1;j<=n;j++)
      if (i!=j)
       {             
               s=a[i]+a[j];
               for (k=1;k<=n;k++)
                if ((a[k]==a[i]+a[j])&&(!used[k]))
                 {                                         
                              ans++;
                              used[k]=true;
                              break; 
                 } 
       }   
    printf("%d\n",ans);
    //fclose(stdin);fclose(stdout);
    return 0;
} 

/**************************************************************
	Problem: 2330
	User: admin
	Language: C++
	Result: Accepted
	Time:50 ms
	Memory:1144 kb
****************************************************************/