#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int c, v0, v1, a, l, count, sum, pr, totalread = 0, days = 0;
  cin >> c >> v0 >> v1 >> a >> l;
  count = c;
  sum = 0;
  while (count > 0) {
    pr = v0 + sum;
    if (pr > v1) pr = v1;
    count = count - pr;
    if (days != 0) {
      count = count + l;
    }
    sum = sum + a;
    days++;
  }
  cout << days;
}
