12321 - Gas Stations/12321.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 pair<int, int> g[10001];
  28
  29 bool compare(const pair<int, int> &a, const pair<int, int> &b) {
  30     if (a.first != b.first) return a.first < b.first;
  31     return a.second > b.second;
  32 }
  33
  34 int main(){
  35     int n;
  36     long long L;
  37     while (scanf("%lld %d", &L, &n) == 2) {
  38         if (L == 0 and n == 0) break;
  39
  40         for (int i = 0; i < n; ++i) {
  41             long long x, r; scanf("%lld %lld", &x, &r);
  42             g[i].first = max(0LL, x - r);
  43             g[i].second = min(L, x + r);
  44         }
  45         sort(g, g + n, compare);
  46
  47         if (g[0].first > 0) {
  48             puts("-1");
  49             continue;
  50         }
  51         int covered = 0;
  52         int ans = 0, i = 0;
  53         while (i < n) {
  54             ans++;
  55             int best = i;
  56             for (int j = i; j < n and g[j].first <= g[i].second; ++j) {
  57                 if (g[j].second > g[best].second) {
  58                     best = j;
  59                 }
  60             }
  61             covered = g[best].second;
  62             if (best == i) break;
  63             i = best;
  64         }
  65         if (covered < L) {
  66             puts("-1");
  67         } else {
  68             assert(ans <= n);
  69             printf("%d\n", n - ans);
  70         }
  71     }
  72     return 0;
  73 }