#include <bits/stdc++.h>
using namespace std;
long long gcd(long long x, long long y) { return x == 0 ? y : gcd(y % x, x); }
long long lcm(long long x, long long y) { return x / gcd(x, y) * y; }
int fx[] = {1, -1, 0, 0};
int fy[] = {0, 0, 1, -1};
int main() {
  int n, L, a, ans = 0, a1[100000], b1[100000];
  cin >> n >> L >> a;
  if (n == 0) {
    cout << L / a;
    return 0;
  }
  for (int i = 1; i <= n; i++) {
    int c, d;
    cin >> a1[i] >> b1[i];
  }
  for (int i = 2; i <= n; i++) {
    int d = a1[i] - (a1[i - 1] + b1[i - 1]);
    ans += d / a;
  }
  ans += (L - (a1[n] + b1[n])) / a;
  ans += ((a1[1]) - 0) / a;
  cout << ans;
  return 0;
}
