#include <bits/stdc++.h>
using namespace std;
int t[100000], l[100000];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int n, L, a;
  cin >> n >> L >> a;
  for (int i = 0; i < n; i++) cin >> t[i] >> l[i];
  for (int k = n / 2; k > 0; k /= 2) {
    for (int i = k; i < n; i++) {
      int j;
      int templ = l[i], tempt = t[i];
      for (j = i; j >= k; j -= k) {
        if (tempt < t[j - k]) {
          t[j] = t[j - k];
          l[j] = l[j - k];
        } else
          break;
      }
      t[j] = tempt;
      l[j] = templ;
    }
  }
  if (n == 0) {
    cout << L / a;
    return 0;
  }
  long long answer = t[0] / a;
  for (int i = 1; i < n; i++) {
    answer += (t[i] - (t[i - 1] + l[i - 1])) / a;
  }
  answer += (L - (t[n - 1] + l[n - 1])) / a;
  cout << answer;
  return 0;
}
