#include <bits/stdc++.h>
using namespace std;
bool isSubSequence(string str1, string str2, int m, int n) {
  int j = 0;
  for (int i = 0; i < n && j < m; i++) {
    if (str1[j] == str2[i]) j++;
  }
  return (j == m);
}
int isPrime(long long n) {
  for (int i = 2; i < sqrt(n); i++)
    if (n % i == 0) return 0;
  return 1;
}
int isSquare(long long n) { return (long long)sqrt(n) * (sqrt(n)) == n; }
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  long long int c, v0, v1, n, i, j, k, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int day = 1, speed = v0 - a, read = 0;
  while (1) {
    if ((speed + a <= v1))
      speed = speed + a;
    else
      speed = v1;
    if (day != 1) read -= l;
    if (read < 0) read = 0;
    read += speed;
    if (read >= c) return cout << day, 0;
    day++;
  }
  return 0;
}
