#include <fstream.h>

ifstream f ("medie.in");
ofstream g ("medie.out");

int n, v[9001];

void read()
{    f >> n;
     for (int i=1; i<=n; i++)
	 f >> v[i];
}

void program()
{    int k=1, nr=1;
     for (int i=k+1; i<n; i++)
	 for (int j=i+1; j<=n; j++)
	     if (2*v[k]==v[i]+v[j]) nr++;
     g << nr;
}

int main()
{   read();
    program();
    return 0;
}