#include <bits/stdc++.h>
using namespace std;
int main() {
  long long n, L, a;
  cin >> n >> L >> a;
  long long t, l;
  long long count1 = 0, count2 = 0;
  for (long long i = 0; i < n; i++) {
    if (i == 0) {
      cin >> t >> l;
      count1 = t + l;
      count2 += (t - 0) / a;
    } else {
      cin >> t >> l;
      count2 += (t - count1) / a;
      count1 = t + l;
    }
  }
  if (count1 < L) {
    count2 += (L - count1) / a;
  }
  cout << count2 << '\n';
}
