#include <bits/stdc++.h>
const int gig = 1e18;
const int gg = 25 * 1e6;
const int md = 1e9 + 7;
const int N = 100050;
const double PI = 3.1415926535897932384626433832795;
using namespace std;
int main() {
  srand(time(NULL));
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  int n, l, a, t1, t2, tl, ans = 0;
  cin >> n >> l >> a;
  tl = 0;
  for (int i = 1; i <= n; i++) {
    cin >> t1 >> t2;
    ans += (t1 - tl) / a;
    tl = t1 + t2;
  }
  ans += (l - tl) / a;
  cout << ans;
}
