program operatii;
var f,g:text;
    v:array[1..1000000] of -1..1000000;
    p,n,i,j,x:longint;

begin
assign(f,'operatii.in');
reset(f);
p:=0;
readln(f,n);
fillchar(v,sizeof(v),-1);
for i:=1 to n do
    begin
      read(f,x);
     { if x>0 then}
          for j:=1 to x do
             if v[j]+1=i then v[j]:=i
                         else begin  v[j]:=i; inc(p); end;
    end;
close(f);
assign(g,'operatii.out');
rewrite(g);
writeln(g,p);
close(g);
end.