12321 - Gas Stations/12321.2.cpp

   1 using namespace std;
   2 #include <algorithm>
   3 #include <iostream>
   4 #include <iterator>
   5 #include <numeric>
   6 #include <sstream>
   7 #include <fstream>
   8 #include <cassert>
   9 #include <climits>
  10 #include <cstdlib>
  11 #include <cstring>
  12 #include <string>
  13 #include <cstdio>
  14 #include <vector>
  15 #include <cmath>
  16 #include <queue>
  17 #include <deque>
  18 #include <stack>
  19 #include <list>
  20 #include <map>
  21 #include <set>
  22
  23 #define foreach(x, v) for (typeof (v).begin() x=(v).begin(); x !=(v).end(); ++x)
  24 #define For(i, a, b) for (int i=(a); i<(b); ++i)
  25 #define D(x) cout << #x " is " << x << endl
  26
  27 const int MAXN = 10010;
  28 pair<int, int> g[MAXN];
  29 int places[2 * MAXN];
  30 int best[2 * MAXN];
  31
  32 bool compare(const pair<int, int> &a, const pair<int, int> &b) {
  33     if (a.first != b.first) return a.first < b.first;
  34     return a.second > b.second;
  35 }
  36
  37 int main(){
  38     int n;
  39     long long L;
  40     while (scanf("%lld %d", &L, &n) == 2) {
  41         if (L == 0 and n == 0) break;
  42         int P = 0;
  43         places[P++] = 0;
  44         for (int i = 0; i < n; ++i) {
  45             long long x, r; scanf("%lld %lld", &x, &r);
  46             g[i].first = max(0LL, x - r);
  47             g[i].second = min(L, x + r);
  48             places[P++] = g[i].first;
  49             places[P++] = g[i].second;
  50         }
  51         places[P++] = L;
  52         sort(places, places + P);
  53         P = unique(places, places + P) - places;
  54
  55         for (int i = 0; i < n; ++i) {
  56             g[i].first = lower_bound(places, places + P, g[i].first) - places;
  57             g[i].second = lower_bound(places, places + P, g[i].second) - places;
  58         }
  59
  60         //sort(g, g + n, compare);
  61         // for (int i = 0; i < n; ++i) {
  62         //     printf("<%d, %d>\n", g[i].first, g[i].second);
  63         // }
  64
  65         for (int i = 0; i < P; ++i) {
  66             best[i] = -1;
  67         }
  68         for (int i = 0; i < n; ++i) {
  69             best[g[i].first] = max(best[g[i].first], g[i].second);
  70         }
  71         for (int i = 1; i < P; ++i) {
  72             best[i] = max(best[i], best[i-1]);
  73         }
  74
  75         // for (int i = 0; i < P; ++i) {
  76         //     printf("best[%d] = %d\n", i, best[i]);
  77         // }
  78
  79
  80         if (g[0].first < 0) {
  81             puts("-1");
  82             continue;
  83         }
  84         int ans = 0;
  85         int cur = 0;
  86         int end = lower_bound(places, places + P, L) - places;
  87         while (cur < end and cur < best[cur]) {
  88             ans++;
  89             cur = best[cur];
  90         }
  91         if (cur < end) {
  92             puts("-1");
  93         } else {
  94             assert(ans <= n);
  95             printf("%d\n", n - ans);
  96         }
  97     }
  98     return 0;
  99 }