#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, L, a;
  cin >> n >> L >> a;
  int t[n];
  int l[n];
  for (int i = 0; i < n; i++) {
    cin >> t[i] >> l[i];
  }
  long long count = t[0] / a;
  for (int i = 0; i < n - 1; i++) {
    long long diff = (t[i + 1] - (t[i] + l[i]));
    count += diff / a;
  }
  long long diff = L - (t[n - 1] + l[n - 1]);
  count += diff / a;
  if (n == 0) {
    cout << L / a << "\n";
  } else
    cout << count << "\n";
  return 0;
}
