#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  int n, L, a;
  cin >> n >> L >> a;
  vector<int> t(n), l(n);
  for (int i = 0; i < n; ++i) {
    cin >> t[i] >> l[i];
  }
  int nob = 0;
  int prevend = 0;
  for (int i = 0; i < n; i++) {
    nob += (t[i] - prevend) / a;
    prevend = t[i] + l[i];
  }
  if (L - prevend >= a) nob += (L - prevend) / a;
  cout << nob;
}
