program ex(f1,f2);
 type
  zn=array [0..10000] of longint;
  ha=array [1..25] of integer;
 var
  a:zn;  v,w:ha;  f1,f2:text; i,j,m,n,k:integer;
  answer:longint;
 begin
  assign(f1,'happy.in'); reset(f1);
  assign(f2,'happy.out'); rewrite(f2);
  readln(f1,n,m);
  for i:=1 to m do
   readln(f1,v[i],w[i]);
  for i:=1 to n do a[i]:=0;
  for i:=1 to m do
   for j:=n downto v[i] do
    if (j-v[i]>=0) and ((a[j-v[i]]+(v[i]*w[i]))>a[j]) then
      a[j]:=a[j-v[i]]+v[i]*w[i];
  answer:=0;
  for i:=1 to n do
   if a[i]>answer then answer:=a[i];
  writeln(f2,answer);
  close(f1);
  close(f2);
 end.