var f1,f2:text;
    n,i,j,k,max,pos:longint;
    sol,aux:int64;
    v:array[0..9000] of longint;
    s,se,ct,v1,ctv:array[1..7000] of longint;


begin
 assign(f1,'medie.in');
 reset(f1);
 assign(f2,'medie.out');
 rewrite(f2);
 readln(f1,n);
 for i:=1 to n do
  readln(f1,v[i]);
 close(f1);
 if n<=200 then
  begin

   //n^3

   for i:=1 to n do
    for j:=1 to n do
     for k:=j+1 to n do
      if (i<>j) and (i<>k) then
       if v[i]<<1=v[j]+v[k] then
        inc(sol);


  end
 else
  begin
   //valmax^2

 for i:=1 to n do
  inc(ctv[v[i]]);
 max:=-1;
 for i:=1 to n do
  if max<v[i] then
   max:=v[i];
 for i:=1 to max do
  begin
   aux:=(ctv[i]*(ctv[i]-1) div 2);
   aux:=aux*(ctv[i]-2);
   inc(sol,aux);
   pos:=1;
   while (i-pos>0) and (i+pos<=max) do
    begin
     inc(sol,ctv[i-pos]*ctv[i+pos]*ctv[i]);
     inc(pos);
    end;
  end;

  end;
 writeln(f2,sol);
 close(f2);
end.