#include <bits/stdc++.h>
using namespace std;
int main(int argc, char const *argv[]) {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int speed = v0, day = 0, page = 0;
  while (page < c) {
    ++day;
    page += speed;
    if (day > 1) page -= l;
    speed += a;
    if (speed > v1) speed = v1;
  }
  cout << day << endl;
  return 0;
}
