program raz;
var n,i,x,y,min,suma,max:integer;
    a: array[1..100] of integer;
begin
readln(n);
for i:= 1 to n do readln(a[i]);
max:=a[n];
suma:=0;
for i:= n-1 downto 1 do begin
max:=max-1;
suma:=suma+(a[n]-max);
end;
writeln(suma);
end.