#include <bits/stdc++.h>
using namespace std;
const long long N = 1e6 + 10;
const long long INF = 1e18;
const long long inf = -1e18;
const long long MOD = 1e9 + 7;
const long long base = 1000 * 1000 * 1000;
int gcd(int a, int b) {
  while (b) {
    a %= b;
    swap(a, b);
  }
  return a;
}
int lcm(int a, int b) { return a / gcd(a, b) * b; }
int n, L, a;
int t[N], l[N];
void solve() {
  cin >> n >> L >> a;
  for (int i = 0; i < n; i++) {
    cin >> t[i] >> l[i];
  }
  int start = 0;
  int ans = 0;
  for (int i = 0; i < n; i++) {
    ans += (t[i] - start) / a;
    start = t[i] + l[i];
  }
  ans += (L - start) / a;
  cout << ans << "\n";
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  solve();
}
