var v:array[1..9000]of longint;
    x,y,m:array[1..7000]of longint;
    i,j,n,s,max:longint;
begin
assign(input,'medie.in');reset(input);
assign(output,'medie.out');rewrite(output);
readln(n);
for i:=1 to n do
        begin
        read(v[i]);
        inc(x[v[i]]);
        end;
for i:=1 to n-1 do
        for j:=i+1 to n do
                begin
                if (v[i]+v[j]) mod 2=0 then
                        inc(y[(v[i]+v[j]) div 2]);
                end;
for i:=1 to n do
        if y[v[i]]<>0 then
                s:=s+y[v[i]]-x[v[i]]+1;
writeln(s);
close(input);close(output);
end.