#include <bits/stdc++.h>
using namespace std;
int c;
int v0;
int v1;
int a;
int l;
int speed;
int main() {
  cin >> c >> v0 >> v1 >> a >> l;
  speed = v0;
  if (speed >= c)
    cout << 1;
  else
    for (int t = 2; t <= c; t++) {
      speed = min(speed + a, v1);
      v0 = v0 - l + speed;
      if (v0 >= c) {
        cout << t;
        return 0;
      }
    }
}
