#include <bits/stdc++.h>
using namespace std;
int n, L, a;
long long T[10000000];
long long len[10000000];
int main() {
  cin >> n >> L >> a;
  int i;
  for (int i = 0; i < n; i++) {
    cin >> T[i];
    cin >> len[i];
  }
  T[n] = (L);
  long long num = 0;
  num = T[0] / a;
  for (long long i = 1; i < n + 1; i++) {
    num += (T[i] - T[i - 1] - len[i - 1]) / a;
  }
  cout << num << endl;
  return 0;
}
