就是noip的第二题！大家帮帮忙，看下那里错拉！
var n,m:longint;
f:array[0..25,0..3000] of qword;
f1:array[1..25,1..2] of longint;
i,j,k,l,o:longint;
begin
assign(input,'happy.in');reset(input);
assign(output,'happy.out');rewrite(output);
readln(n,m);
for i:=1 to m do readln(f1[i,1],f1[i,2]);
fillchar(f,sizeof(f),0);
for i:=1 to m do begin
for j:=0 to n do f[i,j]:=f[i-1,j];
for j:=f1[i,1] to n do if f[i-1,j-f1[i,1]]+f1[i,1]*f1[i,2]>f[i,j] then f[i,j]:=f[i-1,j-f1[i,1]]+f1[i,1]*f1[i,2];
end;
writeln(f[m,n]);
close(input);close(output);
end.
