#include <bits/stdc++.h>
using namespace std;
int main() {
  long long c, w, v, a, l, pages, days = 1;
  cin >> c >> v >> w >> a >> l;
  pages = v;
  while (pages < c) {
    v += a;
    if (v > w) v = w;
    pages = pages - l + v;
    days++;
  }
  cout << days;
}
