#include <bits/stdc++.h>
using namespace std;
int ar[1000];
int main() {
  int i, j, k, c, v1, v2, a, l, m, s, n, ans = 1, w = 0;
  cin >> c >> v1 >> v2 >> a >> l;
  m = v1;
  while (v1 < c) {
    v1 -= l;
    if (v1 < 1) v1 = 1;
    w++;
    if (m + (a * w) <= v2) {
      v1 += (a * w) + m;
    } else
      v1 += v2;
    if (v1 >= c) {
      ans++;
      break;
    }
    ans++;
  }
  cout << ans << endl;
  return 0;
}
