#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 10;
int n, L, a;
int t[N], l[N];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  srand(time(NULL));
  cin >> n >> L >> a;
  int res = 0;
  for (int i = 1; i <= n; i++) {
    cin >> t[i] >> l[i];
    int len = t[i] - (t[i - 1] + l[i - 1]);
    res += len / a;
  }
  res += (L - (t[n] + l[n])) / a;
  cout << res;
}
