#include <bits/stdc++.h>
using namespace std;
int main() {
  int c, v0, v1, a, l;
  cin >> c >> v0 >> v1 >> a >> l;
  int count = 0;
  int i = 1;
  if (c == v0) {
    count++;
    cout << count;
  } else {
    c = c - v0;
    count++;
    while (c > 0) {
      if ((v0 + (i * a)) <= v1) {
        c = c - (v0 + (i * a) - l);
        count++;
        i++;
      } else {
        c = c - v1 + l;
        count++;
      }
    }
    cout << count;
  }
  return 0;
}
