program t1203;
const p=30000;
var a:array[1..p,1..2]of integer;
    b:array[0..30000]of integer;
    n,i,j,i1,t:integer;
procedure qsort(l,r:integer);
var j,i,x,y:integer;
begin
     i:=l;j:=r;
     x:=a[(l+r) div 2,2];
     repeat
           while a[i,2]<x do inc(i);
           while a[j,2]>x do dec(j);
           if i<=j then begin
              y:=a[i,2];a[i,2]:=a[j,2];a[j,2]:=y;
              y:=a[i,1];a[i,1]:=a[j,1];a[j,1]:=y;
              inc(i);dec(j);
              end;
     until i>j;
     if l<j then qsort(l,j);
     if i<r then qsort(i,r);
     end;
begin
     readln(n);
     for i:=1 to n do read(a[i,1],a[i,2]);
     qsort(1,n);
     b[0]:=0; b[1]:=0; j:=1;
     for i:=1 to n do
         begin
         i1:=j;
         for j:=i1 to a[i,2] do b[j]:=b[i1];
         if b[a[i,1]-1]+1>b[a[i,2]] then b[a[i,2]]:=b[a[i,1]-1]+1;
         end;
     writeln(b[a[n,2]]);
end.
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I can pass many inputs in my computer, But still WA