JSOI地铁换票 （贪心）

简单贪心即可。

 type arr=array[..] of longint;
 var a,b:arr;
     i,n,m,sum:longint;
  procedure sort(var a:arr;l,r: longint);
       var
          i,j,x,y: longint;
       begin
          i:=l;
          j:=r;
          x:=a[(l+r) div ];
          repeat
            while a[i]<x do
             inc(i);
            while x<a[j] do
             dec(j);
            if not(i>j) then
              begin
                 y:=a[i];
                 a[i]:=a[j];
                 a[j]:=y;
                 inc(i);
                 j:=j-;
              end;
          until i>j;
          if l<j then
            sort(a,l,j);
          if i<r then
            sort(a,i,r);
       end;
 begin
     readln(n,m);
     for i:= to m do readln(a[i],b[i]);
     sort(a,,m);
     sort(b,,m);
     sum:=;
     for i:= to m do sum:=sum+abs(a[i]-b[i]);
     writeln(sum);

 end.