#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, a;
  cin >> n >> l >> a;
  int x, y;
  int cnt = 0;
  int pre = 0;
  for (int i = 0; i < n; i++) {
    cin >> x >> y;
    int k = (x - pre) / a;
    cnt += k;
    pre = x + y;
  }
  int k = (l - pre) / a;
  cnt += k;
  cout << cnt;
  return 0;
}
