MUABI pascal

type loai = record
	a,b:longint;
end;
var p:array[1..1000] of loai;
        n,m:longint;

procedure nhap;
var i:longint;
begin
        readln(n,m);
        for i:=1 to m do
                read(p[i].a,p[i].b);
end;

procedure bbsort;
var     i,j:longint;
        tam:loai;
begin
        for i:=1 to m-1 do
		for j:=1 to m-i do
			if p[j].b<p[j+1].b then
                        begin
                                tam:=p[j];
                                p[j]:=p[j+1];
                                p[j+1]:=tam;
                        end;
end;

procedure giai;
var i,hop,bi:longint;
begin
        bbsort;
       // for i:=1 to m do writeln(p[i].a,' ',p[i].b);
        hop:=0;bi:=0;
        for i:=1 to m do
	        if hop + p[i].a > N then
                begin
		        //hop:=hop+(N - p[i].a);
		        bi:=bi+(N-hop)*p[i].b;
		        break;
                end
	        else
                begin
		        hop:=hop+p[i].a;
		        bi:=bi+p[i].a*p[i].b;
                end;
        write(bi);
end;

begin
        assign(input,'muabi.inp');reset(input);
        assign(output,'muabi.out');rewrite(output);
        nhap;
        giai;
end.