var a:array[0..10000,1..2]of longint; i,j,n,ans,s:longint;begin readln(s,n); for i:=1 to n do read(a[i,1],a[i,2]); for i:=1 to n-1 do for j:=i+1 to n do if a[i,1]>a[j,1] then begin a[0]:=a[i]; a[i]:=a[j]; a[j]:=a[0]; end; i:=0; while (s<>0) do begin inc(i); if s-a[i,2]>=0 then begin s:=s-a[i,2]; ans:=ans+a[i,1]*a[i,2]; end else begin ans:=ans+s*a[i,1]; s:=0; end; end; write(ans);end.
