#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  int n, l, a;
  cin >> n >> l >> a;
  int tmp = 0, ans = 0;
  for (int i = 1; i <= n; i++) {
    int x, y;
    cin >> x >> y;
    ans += (x - tmp) / a;
    tmp = x + y;
  }
  ans += (l - tmp) / a;
  cout << ans;
  return 0;
}
