#include <bits/stdc++.h>
using namespace std;
const double EPS = 1e-9;
const int mod = 1e9 + 7;
template <typename T>
T gcd(T a, T b) {
  T c;
  while (b) {
    c = b;
    b = a % b;
    a = c;
  }
  return a;
}
template <typename T>
T powmod(T a, T b) {
  T res = 1;
  a %= mod;
  while (b) {
    if (b & 1) res = res * (a % mod);
    a = a * (a % mod);
    b >>= 1;
  }
  return res;
}
void Solve() {
  int n, L, a;
  cin >> n >> L >> a;
  int ans = 0;
  int tim = 0;
  for (int i = 0; i < n; ++i) {
    int t, l;
    cin >> t >> l;
    ans += (t - tim) / a;
    tim = t + l;
  }
  ans += (L - tim) / a;
  cout << ans << '\n';
}
int main() {
  std::ios::sync_with_stdio(false);
  std::cin.tie(nullptr);
  Solve();
}
