#include <bits/stdc++.h>
using namespace std;
struct IntCmp {
  bool operator()(const std::pair<std::string, int> &lhs,
                  const std::pair<std::string, int> &rhs) {
    return lhs.second > rhs.second;
  }
};
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long c, v0, v1, a, l, cnt = 0, days = 0, speed;
  cin >> c >> v0 >> v1 >> a >> l;
  while (cnt < c) {
    days++;
    if (days == 1) {
      speed = v0;
      cnt += speed;
      continue;
    } else
      speed += a;
    if (speed > v1) speed = v1;
    cnt += speed - l;
  }
  cout << days;
  return 0;
}
