#include <bits/stdc++.h>
using namespace std;
const int inf = (int)1e9;
const int mod = inf + 7;
int c, v0, v1, a, l, cnt;
int main() {
  cin >> c >> v0 >> v1 >> a >> l;
  int ind = 0;
  while (true) {
    if (cnt >= c) {
      break;
    }
    if (ind == 0) {
      if (v0 <= v1) {
        cnt += v0;
      } else {
        cnt += v1;
      }
    } else {
      int t = v0 + ind * a;
      cnt -= l;
      if (t <= v1) {
        cnt += t;
      } else {
        cnt += v1;
      }
    }
    ind++;
  }
  cout << ind;
  return 0;
}
