#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int days = 0, ans = 0, val;
  while (true) {
    val = (v0 + (days)*a);
    ans += (val > v1) ? v1 : val;
    days++;
    if (days > 1) ans -= l;
    if (ans >= c) break;
  }
  cout << days << '\n';
  return 0;
}
