#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, l, k, num;
  int a, b, c;
  cin >> n >> l >> k;
  for (int i = 0; i < n; i++) {
    cin >> a >> c;
    num += (a - b) / k;
    b = a + c;
  }
  num += (l - b) / k;
  cout << num << endl;
}
