C++ :

#include<cstdio>
#include<algorithm>
using namespace std;
unsigned long long ans,n,a[1000039];
int main(){
	register int i;
	scanf("%llu",&n);
	for(i=1;i<=n;i++) scanf("%llu",&a[i]);
	sort(a+1,a+n+1);
	for(i=1;i<=n;i+=2) ans+=a[i];
	printf("%llu",ans);
}

Pascal :

var a:array[1..1000000] of longint;
    s,ss:string;
    i,n:longint;

procedure qsort(l,r:longint);
var i,j,t,mid:longint;
begin
  i:=l; j:=r; mid:=a[(i+j) div 2];
  repeat
    while a[i]<mid do inc(i);
    while a[j]>mid do dec(j);
    if i<=j then
    begin
      t:=a[i]; a[i]:=a[j]; a[j]:=t;
      inc(i); dec(j);
    end;
  until i>j;
  if l<j then qsort(l,j);
  if i<r then qsort(i,r);
end;

procedure gjd(s1,s2:string);
var a,b,c:array[1..256] of longint;
    la,lb,lc,t,i:longint;
    ss,sss:string;
begin
  fillchar(a,sizeof(a),0);
  fillchar(b,sizeof(b),0);
  fillchar(c,sizeof(c),0);
  la:=length(s1);
  for i:=1 to la do
    a[la+1-i]:=ord(s1[i])-ord('0');
  lb:=length(s2);
  for i:=1 to lb do
    b[lb+1-i]:=ord(s2[i])-ord('0');
  if la>lb then lc:=la
    else lc:=lb;
  t:=0;
  for i:=1 to lc do
  begin
    c[i]:=a[i]+b[i]+t;
    t:=c[i] div 10;
    c[i]:=c[i] mod 10;
  end;
  if t>0 then
  begin
    inc(lc);
    c[lc]:=t;
  end;
  ss:=''; sss:='';
  for i:=lc downto 1 do
  begin
    str(c[i],ss);
    sss:=sss+ss;
    ss:='';
  end;
  s:=sss;
end;

begin
  readln(n);
  for i:=1 to n do
    readln(a[i]);
  qsort(1,n);
  s:='0';
  for i:=n downto 1 do
  begin
    str(a[i],ss);
    if i mod 2=1 then gjd(s,ss);
  end;
  writeln(s);
end.