PROGRAM medic(input,output,f1,f2);
type
 fu=array [1..1000] of longint;
 var
  f1,f2:text;
  m,t:longint;
  a,b,c:fu;
  time,money,i:longint;
  procedure fuqiang(a,b:fu);
   begin
     if a[i]>t then exit;
      if a[i]<=t then
       begin
         time:=a[i]+time;
          if time>t then exit;
           if time<=t then
            money:=money+b[i];
               end;
            end;
begin
 assign(f1,'medic.in');
 assign(f2,'medic.out');
 reset(f1);
 rewrite(f2);
 readln(f1,t,m);
 time:=0;
 money:=0;
 for i:=1 to m do begin
  read(f1,a[i]);
  read(f1,b[i]);
  fuqiang(a,b);
  end;
  writeln(f2,money);
  close(f1);
  close(f2);
  end.