#include <bits/stdc++.h>
using namespace std;
long long power(long long x, long long n) {
  long long m = 1;
  for (int i = 0; i < n; i++) {
    m = ((m % 1000000007) * (x % 1000000007)) % 1000000007;
  }
  return m;
}
long long power2(long long x, unsigned long long y) {
  long long temp;
  if (y == 0) return 1;
  temp = power(x, y / 2);
  if (y % 2 == 0)
    return ((temp % 1000000007) * (temp % 1000000007)) % 1000000007;
  else
    return ((x % 1000000007) * (temp % 1000000007) * (temp % 1000000007)) %
           1000000007;
}
int main() {
  ios::sync_with_stdio(false);
  long long i, j, k, l, x, y, n, m, a, sum = 0, mx = LONG_LONG_MIN,
                                       mn = LONG_LONG_MAX;
  cin >> n >> l >> a;
  vector<long long> v;
  v.push_back(0);
  for (i = 0; i < n; i++) {
    cin >> x >> y;
    v.push_back(x);
    v.push_back(x + y);
  }
  v.push_back(l);
  int len = v.size();
  long long ans = 0;
  for (i = 1; i < len; i += 2) {
    ans += (v[i] - v[i - 1]) / a;
  }
  cout << ans;
  return 0;
}
