program lazy;
var i,j,k,n,p:longint;
  a,b:array[1..10000] of longint;
  f:array[1..10001] of longint;
procedure sort(l,r:longint);
var
  i,j,x,y: longint;
begin
  i:=l;
  j:=r;
  x:=a[(l+r) div 2];
  repeat
    while a[i]>x do inc(i);
    while x>a[j] do dec(j);
   if not(i>j)
   then begin
     y:=a[i];
     a[i]:=a[j];
     a[j]:=y;
     y:=b[i];
     b[i]:=b[j];
     b[j]:=y;
     inc(i);
     j:=j-1;
   end;
  until i>j;
  if l<j then sort(l,j);
  if i<r then sort(i,r);
end;
begin
  assign(input,'lazy.in');
  assign(output,'lazy.out');
  reset(input);
  rewrite(output);
  read(n,k);
  for i:=1 to k do
    read(a[i],b[i]);
  sort(1,k);
  p:=1;
  fillchar(f,sizeof(f),0);
  for i:=n downto 1 do
    if a[p]=i
      then begin
        f[i]:=f[a[p]+b[p]];
        inc(p);
        while a[p]=i do
        begin
          if f[i]<f[a[p]+b[p]]
            then f[i]:=f[a[p]+b[p]];
          inc(p);
        end;
      end
    else f[i]:=f[i+1]+1;
  writeln(f[1]);
  close(input);
  close(output);
end.