program prvi;

uses crt;

var
  n, i, pr, uk:longint;
  a:array of longint;

begin
  clrscr;
  readln(n);
  setlength(a, n);
  uk:=0;
  for i:=1 to n do
    readln(a[i-1]);
  for i:=n-1 downto 0 do
    begin
    if i<n-1 then
      pr:=a[i+1]
    else
      continue;
    if pr<=a[i] then
      begin
      uk:=uk+(a[i]-pr)+1;
      a[i]:=pr-1;
      end;
    end;
  writeln(uk);
  //readln;
end.