#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  scanf("%d %d %d %d %d", &c, &v0, &v1, &a, &l);
  int sum = 0;
  int day = 0;
  int k = v0;
  while (sum < c) {
    if (day == 0) {
      sum += k;
    } else
      sum = sum + k - l;
    k += a;
    if (k > v1) k = v1;
    day++;
  }
  printf("%d\n", day);
  return 0;
}
