#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, a;
  cin >> n >> l >> a;
  int ans = 0, prev = 0;
  while (n--) {
    int x, y;
    cin >> x >> y;
    ans += (x - prev) / a;
    prev = x + y;
  }
  ans += (l - prev) / a;
  cout << ans;
}
