1 /**************************************************************
 2     Problem: 1747
 3     User: rausen
 4     Language: C++
 5     Result: Accepted
 6     Time:20 ms
 7     Memory:916 kb
 8 ****************************************************************/
 9
10 #include <cstdio>
11 #include <algorithm>
12 #include <queue>
13
14 using namespace std;
15 const int N = 1e4 + 5;
16
17 inline int read();
18
19 struct data {
20     int w, add;
21
22     inline void get() {
23         w = read(), add = read();
24     }
25
26     inline bool operator < (const data &d) const {
27         return w > d.w;
28     }
29 } a[N];
30
31 int n, tot, ans;
32 priority_queue <int> h;
33
34 int main() {
35     int i;
36     n = read();
37     for (i = 1; i <= n; ++i) a[i].get();
38     a[0].w = read(), tot = read();
39     sort(a + 1, a + n + 1);
40     a[n + 1].w = 0;
41     for (i = 1; i <= n + 1; ++i) {
42         tot -= (a[i - 1].w - a[i].w);
43         while (tot < 0 && !h.empty()) {
44             ++ans;
45             tot += h.top(), h.pop();
46         }
47         if (tot < 0) {
48             puts("-1");
49             return 0;
50         }
51         h.push(a[i].add);
52     }
53     printf("%d\n", ans);
54     return 0;
55 }
56
57 inline int read() {
58     static int x;
59     static char ch;
60     x = 0, ch = getchar();
61     while (ch < '0' || '9' < ch)
62         ch = getchar();
63     while ('0' <= ch && ch <= '9') {
64         x = x * 10 + ch - '0';
65         ch = getchar();
66     }
67     return x;
68 }