#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int day = 1;
  int now = v0;
  int speed = v0;
  while (now < c) {
    day++;
    speed = min(speed + a, v1);
    now += speed - l;
  }
  cout << day;
  return 0;
}
