var i,n,max,s,j:longint;
    v:array[1..10000] of longint;
    f,g:text;
begin
assign(f,'operatii.in');reset(f);
assign(g,'operatii.out');rewrite(g);
read(f,n);
for i:=1 to n do read(f,v[i]);
close(f);
i:=2;
max:=0;
s:=0;
while i<=n do begin
              if v[i]>=v[i-1] then max:=v[i]
                              else begin
                                   j:=i;
                                   while (v[j]>=v[j+1]) or (v[j]<>0) do begin
                                                                        inc(i);
                                                                        inc(j);
                                                                        end;
                                   s:=s+max;
                                   max:=0;
                                   end;
              inc(i);
              end;
write(g,s+max);
close(g);
end.