1 #include <iostream>
 2 #include <stdio.h>
 3 #include <queue>
 4 #include <vector>
 5 #include <algorithm>
 6 using namespace std;
 7 struct Node{int L,P;};
 8 struct CMP_L{bool operator()(Node a,Node b){return a.L<b.L;}};
 9 struct CMP_P{bool operator()(Node a,Node b){return a.P<b.P;}};
10 int main()
11 {
12     int N,i,j,L,P,Begin,Dic,Times;
13     Node NUM;
14     while(scanf("%d",&N)!=EOF)
15     {
16         priority_queue<Node,vector<Node>,CMP_L>ID_L;
17         priority_queue<Node,vector<Node>,CMP_P>ID_P;
18         for(i=0;i<N;i++)
19         {
20             scanf("%d%d",&NUM.L,&NUM.P);
21             ID_L.push(NUM);
22         }
23         scanf("%d%d",&L,&P);
24         NUM.L=0;NUM.P=0;
25         ID_L.push(NUM);
26 
27         Begin=L;Times=0;
28         while(!ID_L.empty())
29         {
30             NUM=ID_L.top();ID_L.pop();
31             Dic=Begin-NUM.L;
32             while(P-Dic<0)
33             {
34                 if(ID_P.empty())
35                 {
36                     Times=-1;
37                     break;
38                 }
39                 P+=ID_P.top().P;
40                 ID_P.pop();
41                 Times++;
42             }
43             if(Times==-1)break;
44             P-=Dic;
45             Begin=NUM.L;
46             ID_P.push(NUM);
47         }
48         printf("%d\n",Times);
49     }
50     return 0;
51 }