#include <bits/stdc++.h>
using namespace std;
bool isFile = 0;
const long long INF = 1e9 + 1234;
const int N = 5e5 + 1234;
const long long MOD = 998244353;
const long long mod = 1e9 + 7;
double EPS = 3e-18;
const double PI = acos(-1);
void faster() {
  if (isFile) {
    freopen(
        ""
        ".in",
        "r", stdin);
    freopen(
        ""
        ".out",
        "w", stdout);
  }
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
}
void solve() {
  int n, L, a;
  cin >> n >> L >> a;
  int tlast = 0;
  int ans = 0;
  bool fl = 0;
  for (int i = 0; i < n; i++) {
    int t, l;
    cin >> t >> l;
    if (!fl) {
      ans += (t - tlast) / a;
      tlast = t + l;
      if (tlast >= L) fl = 1;
    }
  }
  if (!fl) {
    ans += (L - tlast) / a;
  }
  cout << ans;
}
int main() {
  faster();
  int t = 1;
  while (t--) {
    solve();
    cout << "\n";
  }
  return 0;
}
