var
  a: array[1..100] of byte;
  n, i: byte;
  sum: longint;

begin
  readln(n);
  for i := 1 to n do
    read(a[i]);
  for i := 1 to n do
    if (i mod 2 = 0) then sum += a[i]; // sum := sum + a[i];
  writeln(sum);
end.