program Project2;

{$APPTYPE CONSOLE}

uses
  SysUtils;
var
  n, i: integer;
  a, sum: int64;
begin
  readln (n);
  sum := 0;
  for i := 1 to n do begin
    read (a);
    sum := sum + (a * (a - 1)) div 2;
  end;
  writeln (sum);
  readln;
  readln;
end.