#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, l, a, x, y, sum, t[1000009], px, py;
  sum = 0;
  px = py = 0;
  cin >> n >> l >> a;
  for (int i = 0; i <= n; i++) {
    if (i == n) {
      t[i] = l - (px + py);
      break;
    }
    cin >> x >> y;
    if (i == 0) {
      t[i] = x - 0;
      px = x;
      py = y;
    } else {
      t[i] = x - (px + py);
    }
    px = x;
    py = y;
  }
  for (int i = 0; i <= n; i++) {
    if (t[i] >= a) {
      sum += (t[i] / a);
    }
  }
  cout << sum << endl;
  return 0;
}
