#include<fstream>
using namespace std;
ifstream cin("bete2.in");
ofstream cout("bete2.out");
unsigned long long  n, contor;long long sir[50005],j;
bool Bsearch ( int val ) {
 
int mid , left = j , right = n;
 
while ( left <= right ) {
mid = ( left + right )/2;
if ( sir[mid] == val ) return 1;
else if ( sir[mid] < val ) left = mid + 1;
else right = mid - 1;
}
 
return 0;
}
void sortare(long long arr[], long long left, long long right)
{
long long i = left, j = right;
long long tmp;
long long pivot = arr[(left + right) / 2];
 
while (i <= j)
{
while (arr[i] < pivot)
i++;
while (arr[j] > pivot)
j--;
if (i <= j)
{
tmp = arr[i];
arr[i] = arr[j];
arr[j] = tmp;
i++;
j--;
}
}
if (left < j)
sortare(arr, left, j);
if (i < right)
sortare(arr, i, right);
}
 
int main()
{
cin>>n;
for(int i=1;i<=n;i++)
cin>>sir[i];
sortare(sir, 1, n);
for(int i=1;i<=n-2;i++)
for(int j=i+1;j<=n-1;j++)
if (Bsearch(sir[i]+sir[j]))
contor++;
cout<<contor;
return 0;
}