program razine;
var n,a,i:integer;
    bodovi:array[1..100] of integer;
begin
     readln(n);
     for i:=1 to n do readln(bodovi[i]);
     a:=0;
     for i:=n downto 2 do if bodovi[i-1]>=bodovi[i] then repeat
                                                          bodovi[i-1]:=bodovi[i-1]-1;
                                                          a:=a+1;
                                                         until bodovi[i]>bodovi[i-1];
     writeln(a);

end.