#include <bits/stdc++.h>
using namespace std;
const double g = 10.0, eps = 1e-9;
const int N = 1000 + 10, maxn = 1000 + 10, inf = 1000000000;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int sum = 0, k = 0;
  while (sum < c) {
    sum += v0;
    if (k) sum -= l;
    k++;
    v0 += a;
    if (v0 > v1) v0 = v1;
  }
  cout << k << endl;
  return 0;
}
