#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6 + 101;
long long n, L, a;
long long t[N], l[N], ans;
signed main() {
  cin >> n >> L >> a;
  for (long long i = 1; i <= n; i++) cin >> t[i] >> l[i];
  for (long long i = 1; i <= n; i++) {
    if (t[i] - (t[i - 1] + l[i - 1]) >= a) {
      ans = ans + (t[i] - (t[i - 1] + l[i - 1])) / a;
    }
  }
  ans = ans + (L - t[n] - l[n]) / a;
  cout << ans << endl;
}
