#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, l, a, q[100001], w[100001], sum = 0, i;
  cin >> n >> l >> a;
  for (i = 0; i < n; i++) {
    cin >> q[i] >> w[i];
  }
  if (n == 0) {
    cout << l / a;
    return 0;
  }
  sum = sum + q[0] / a;
  for (i = 1; i < n; i++) {
    sum = sum + (q[i] - w[i - 1] - q[i - 1]) / a;
  }
  sum = sum + (l - w[n - 1] - q[i - 1]) / a;
  cout << sum;
}
