Stacked box problem

Stacked box problem
Time limit:10000ms Memory limit:65536k
Total submit:315 accepted:117
Case Time Limit:1000ms
Description
A port has a number of containers, numbered 1 to N, respectively. The size of each box is the same, now to be some of the containers stacked up, container stacking rules are as follows:
1) Only one container can be stacked directly on each container.
2) containers with smaller numbers cannot be placed on larger containers.
3) Each container is given its own weight and weight, and the weight of all containers on top of each container must not exceed the weight of the container.
You are now required to program and select the most numerous containers, so that they can be stacked up to meet the above conditions, that is, the requirement to fold as high as possible.
Input
The first line is a positive integer n, representing a total of n containers (1≤n≤1000).
The following is a total of n rows, two positive integers per line, separated by a space, respectively, each container's own weight and the amount of weight, two numbers are less than or equal to 3000.
Output
Outputs the total number of containers that can be stacked. The run time does not exceed 10 seconds to go.
Sample Input
5
19 15
7 13
5 7
6 8
1 2
Sample Output

4

Var
 a,w:array[1..1000]of longint;
 f:array[0..1000,0..6000]of longint;
 i,j,n,max1: Longint;
Function Max (a,b:longint): Longint;
 begin
  If A>b then exit (a);
  exit (b);
 end;
Begin
 read (n);
&N Bsp;for i:=1 to N does
  READLN (W[i],a[i]);
 f[n,w[n]]:=1;
 for i:=n-1 downto 1 do
 begi N
  f[i]:=f[i+1];
  for j:=0 to A[i] do
   f[i,j+w[i]]:=max (F[i+1,j]+1,f[i,j+w[i]]);
 end;
 max1:=0;
 for i:=0 to 6000 do
  Max1:=max (Max1,f[1,i]);
 write (MAX1);
End.