#include <bits/stdc++.h>
using namespace std;
long c, v, v1, a, l;
int main() {
  cin >> c >> v >> v1 >> a >> l;
  long sum = v, days = 1, num = v;
  while (sum < c) {
    num = min(v1, num + a);
    sum += num - l;
    days++;
  }
  cout << days;
}
