#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  int c = 0, v0 = 0, v1 = 0, a = 0, l = 0;
  cin >> c >> v0 >> v1 >> a >> l;
  int lec = v0;
  int days = 1;
  while (lec < c) {
    lec -= l;
    if ((v0 + (a * days)) > v1) {
      lec = lec + v1;
    } else {
      lec = lec + v0 + (a * days);
    }
    ++days;
  }
  cout << days << endl;
  return 0;
}
