var
i,j,n,m:longint;
v,p:array[1..25] of longint;
f:array[0..25,0..30000] of longint;

begin
assign(input,'p1317.in');
reset(input);
readln(n,m);
for i:=1 to m do
  read(v[i],p[i]);
for i:=0 to n do
  f[0,i]:=0;
for i:=1 to m do
  for j:=0 to n do
    begin
      f[i,j]:=f[i-1,j];
      if (j>=v[i]) and (f[i-1,j-v[i]]+v[i]*p[i]>f[i,j]) then
          f[i,j]:=f[i-1,j-v[i]]+v[i]*p[i];
    end;
write(f[m,n]);
end.
//delphi/7198