const f = 'medie.in'; g = 'medie.out';
var
    a, sol : array[1..9005] of longint;
    j, n, i, s, k : longint;

begin
    assign( input,f ); reset( input );
    assign( output,g ); rewrite( output );
    readln( n );
    for i := 1 to n do
        begin
        readln(a[i]);
        inc( sol[a[i]] );
        end;
    k := 0;
    for i :=2 to n do
        for j :=1 to i-1 do
            begin
            s := a[i] + a[j];
            if s mod 2=0 then
                begin
                k := k + sol[s div 2];
                if a[i] = a[j] then k := k -2;
                end;
            end;
    writeln( k );
end.