#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 1;
const int inf = 1e9;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);
  long long int c, v, v1, l, a, i, j;
  cin >> c >> v >> v1 >> a >> l;
  long long int cnt = 0, curr = 0, prev = 0;
  cnt++;
  c -= v;
  curr = v;
  while (c > 0) {
    cnt++;
    curr += a;
    curr = min(curr, v1);
    c -= (curr - l);
  }
  cout << cnt, cout << "\n";
  return 0;
}
