#include <stdio.h>
#define infile "medie.in"
#define outfile "medie.out"
#define nmax 9001

int n, v[nmax], i, j, k;
long rez;

void readdata()
{
 FILE *fin=fopen(infile, "r");
 fscanf(fin, "%d\n", &n);
 for (i=0; i<n; i++)
     fscanf(fin, "%d\n", &v[i]);
 fclose(fin);
}

int ok(int a, int b, int c)
{
 if (2*a==b+c) return 1;
 return 0;
}

void solve()
{
 for (i=0; i<n; i++)
     for (j=0; j<n-1; j++)
	 for(k=j+1; k<n; k++)
	     if (ok(v[i],v[j],v[k]) && i!=j && i!=k) rez++;
}

void writedata()
{
 FILE *fout=fopen(outfile, "w");
 fprintf(fout, "%ld", rez);
 fclose(fout);
}

int main()
{
 readdata();
 solve();
 writedata();
 return 0;
}