program razine;
var a,b,c,d,e,f:longint;
arr:array[1..100] of longint;
begin
d:=0;
readln(a);
for b:=1 to a do readln(arr[b]);
for b:=a downto 2 do begin
if arr[b]<=arr[b-1] then begin
c:=arr[b-1]-arr[b]+1; d:=d+c;
arr[b-1]:=arr[b]-1;
end;end;
writeln(d);
end.