#include <bits/stdc++.h>
using namespace std;
pair<long, long> arr[100001];
int main() {
  long long L, a;
  int n;
  scanf("%d %lld %lld", &n, &L, &a);
  long long lastfinish = 0;
  long long ans = 0;
  for (int i = 0; i < n; i++) {
    long long t1, t2;
    scanf("%lld %lld", &arr[i].first, &arr[i].second);
  }
  sort(arr, arr + n);
  for (int i = 0; i < n; i++) {
    if (arr[i].first - lastfinish >= a) {
      ans += (arr[i].first - lastfinish) / a;
    }
    lastfinish = arr[i].first + arr[i].second;
  }
  if (L - lastfinish >= a) ans += (L - lastfinish) / a;
  printf("%lld\n", ans);
  return 0;
}
