program operatii;
var f,g:text;
    v:array[1..1000000]of 0..1000001;
    n,i,min,j,c,co:longint;
begin
assign(f,'operatii.in');
assign(g,'operatii.out');
reset(f);
rewrite(G);
read(f,n);
for i:=1 to n do
  read(f,v[i]);
i:=1;
while (i<=n)do
  begin
    if (v[i]=0)then
      begin
        min:=maxlongint;
        for j:=i+1 to n do
          begin
            if (v[j]=0)then break;
            if (v[j]<min)then min:=v[j];
          end;
        c:=c+min;
      end;
    while (v[i]=v[i+1]) do
      inc(i);
    if (v[i]>min)then c:=c+v[i]-min;
    inc(i);
  end;
write(g,c);
close(f);
close(g);
end.