#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  int vs, days = 0, npr = 0;
  cin >> c;
  cin >> v0;
  cin >> v1;
  cin >> a;
  cin >> l;
  vs = v0;
  c -= v0;
  days++;
  while (c > 0) {
    if (vs < v1) {
      vs += a;
      if (vs > v1) vs = v1;
      c -= vs - l;
      days++;
    } else {
      c -= v1 - l;
      days++;
    }
  }
  cout << days;
  return 0;
}
