program medieeee;   
type vec=array[1..9000] of integer;   
var a,b:vec;   
    n,i,j,k,s,e:integer;   
    nr:longint;   
    f,g:text;   
begin  
assign(f,'medie.in');assign(g,'medie.out');reset(f);rewrite(G);   
readln(f,n);   
for i:=1 to n do begin  
readln(f,a[i]);b[a[i]]:=b[a[i]]+1;;end;   
for i:=1 to n-1 do  
for j:=i+1 to n do begin  
s:=(a[i]+a[j]) div 2;   
if a[i]+a[j]=s*2 then begin  
e:=b[s];   
if a[i]=s then e:=e-1;   
if a[j]=s then e:=e-1;   
nr:=nr+e;   
end;   
end;   
write(g,nr);   
close(f);close(g);end.