#include <bits/stdc++.h>
using namespace std;
int n, L, a;
void read() {
  int x = 0, y = 0, lx = 0, ly = 0, ans = 0;
  scanf("%d %d %d", &n, &L, &a);
  for (int i = 0; i < n; i++) {
    scanf("%d %d", &x, &y);
    ans += (x - (lx + ly)) / a;
    lx = x;
    ly = y;
  }
  ans += (L - (x + y)) / a;
  printf("%d\n", ans);
}
int main() {
  read();
  return 0;
}
