#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int s = v0;
  int rem = c;
  int reread = 0;
  int days = 0;
  while (1) {
    days++;
    if (c + reread <= s) {
      break;
    }
    c = c - s + reread;
    s = s + a;
    reread = l;
    s = min(s, v1);
  }
  cout << days;
  return 0;
}
