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(y[v[i]]);
        if v[i]>max then
                max:=v[i];
        end;
{if n<max then
        begin}
        for i:=1 to n-1 do
                for j:=i+1 to n do
                          if (v[i]+v[j])mod 2=0 then
                                inc(m[(v[i]+v[j])div 2]);
        for i:=1 to n do
                s:=s+m[v[i]];
        writeln(s);
        close(output);
        halt;
{        end;
for i:=1 to max do
        begin
        if y[i]<>0 then
                for j:=1 to max do
                        if (y[j]<>0)and((i+j)mod 2=0) then
                                if j<>i then
                                        inc(m[(i+j)div 2],y[i]*y[j])
                                else
                                        inc(m[(i+j)div 2],y[i]*(y[i]-1)*(y[i]-2));
        end;
s:=0;
for i:=1 to max do
        s:=s+m[i]*ord(y[i]>0);
writeln(s div 2);
close(input);close(output);}
end.