很久之前做的了，已经想不起思路了，容我日后补充。

 1 #include <stdio.h>
 2 #include <deque>
 3 #include <algorithm>
 4 #define inf 1000000000
 5 using namespace std;
 6 int n, L, a, b, cnt = 1;
 7 int f[1000010];
 8 bool d[1000010];
 9 struct node
10 {
11     int s, e;
12 }p[1001], P[1001];
13 bool cmp(const node &a, const node &b)
14 {
15     if(a.s != b.s) return a.s < b.s;
16     else return a.e > b.e;
17 }
18 int main()
19 {
20     int beg,end,i,j,t;
21     scanf("%d%d%d%d", &n, &L, &a, &b);
22     for(i = 1; i <= n; ++i)
23         scanf("%d%d", &p[i].s, &p[i].e);
24     sort(p + 1, p + n + 1, cmp);
25     P[1] = p[1];
26     for(i = 2; i <= n; ++i)
27         if(p[i].e > p[i - 1].e)
28             P[++cnt] = p[i];
29     beg = end = 0;
30     for(i = 1; i <= cnt; ++i)
31     {
32         if(P[i].s >= end)
33         {
34             for(j = beg + 1; j <= end - 1; ++j)
35                 d[j] = 1;
36             beg = P[i].s;
37             end = P[i].e;
38         }
39         else end = P[i].e;
40     }
41     for(j = beg + 1; j <= end - 1; ++j)
42         d[j] = 1;
43     f[0] = 0;
44     deque<int>Q;
45     for(i = 1; i <= L; ++i)
46     {
47         f[i] = inf;
48         while(!Q.empty() && Q.front() < i - 2 * b)
49             Q.pop_front();
50         if(!d[i] && i % 2 == 0)
51         {
52             if(!Q.empty())
53                 f[i] = min(f[i], f[Q.front()] + 1);
54         }
55         t = i - 2 * a + 1;
56         if(t >= 0 && !d[t] && t % 2 == 0)
57         {
58             while(!Q.empty() && f[t] < f[Q.back()])
59                 Q.pop_back();       
60             Q.push_back(t);
61         }
62     }
63     if(f[L] == inf) puts("-1");
64     else printf("%d\n", f[L]);
65     return 0;
66 }