#include <stdio.h>
//#include <time.h>
//#include <dos.h>


int v[7001];
int a[9001];


int main()
{
        register long rez=0;
	FILE *f, *g;
	f=fopen("medie.in", "r");
	g=fopen("medie.out", "w");
	//clock_t start, end;
	//start = clock();
	int n, i, j;
	register int aux;
	fscanf (f, "%d", &n);
	for (i=0; i<n; i++)
		{fscanf(f, "%d", &a[i]);
		v[a[i]]++;  }
	fclose(f);
	for (i=0; i<n-1; i++)
		for (j=i+1; j<n; j++)
			if (a[i]==a[j])
				rez+=v[a[i]]-2;
			else
				{ aux=a[i]+a[j];
				if (aux%2==0)
						if (v[aux/2]) rez+=v[aux/2];
				}
	fprintf(g, "%ld\n", rez);
	//end = clock();
	//fprintf(g, "Time: %f\n", (double)(end - start) / CLK_TCK);
        //fprintf(g, "\n");
	fclose(g);
	return 0;
}