#include <bits/stdc++.h>
using namespace std;
const int INF = 0x3f3f3f3f;
const int MAXINT = 2147483647;
const int MAXN = 100005;
int n, l, a, st[MAXN], ed[MAXN], ans;
int main() {
  int last = 0, i, x, s;
  scanf("%d%d%d", &n, &l, &a);
  for (i = 1; i <= n; i++) {
    scanf("%d%d", &st[i], &x);
    s = st[i] - last;
    s = s / a;
    ans += s;
    ed[i] = st[i] + x;
    last = ed[i];
  }
  s = l - last;
  s = s / a;
  ans += s;
  printf("%d", ans);
  return 0;
}
