#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long n, L, a;
  cin >> n >> L >> a;
  if (n == 0) {
    cout << L / a << "\n";
  } else {
    long long x[n], b[n];
    long long ans = 0;
    for (long long i = 0; i < n; i++) {
      cin >> x[i] >> b[i];
      if (i > 0) ans += (x[i] - (b[i - 1] + x[i - 1])) / a;
    }
    ans += (x[0] - 0) / a;
    ans += (L - (x[n - 1] + b[n - 1])) / a;
    cout << ans << "\n";
  }
  return 0;
}
