#include <bits/stdc++.h>
using namespace std;
int c, v0, v1, a, l;
int get_today(int t) {
  int ans = v0 + a * (t - 1);
  if (ans >= v1) ans = v1;
  if (t >= 2) {
    int sub = l;
    sub = min(sub, v0 + a * (t - 2));
    ans -= sub;
  }
  return ans;
}
int main() {
  cin >> c >> v0 >> v1 >> a >> l;
  int ans = 1, sum = 0;
  for (;; ++ans) {
    sum += get_today(ans);
    if (sum >= c) break;
  }
  cout << ans << endl;
}
