#include <bits/stdc++.h>
using namespace std;
const int INF = 0x7FFFFFFF;
void Filework(void) {
  freopen(".in", "r", stdin);
  freopen(".out", "w", stdout);
}
int c, v0, v1, l, a;
int d;
int total;
int main() {
  int i;
  scanf("%d%d%d%d%d", &c, &v0, &v1, &a, &l);
  total = 1;
  while (true) {
    d++;
    if (d != 1) {
      total -= l;
    }
    total += (v0 - 1);
    v0 += (a);
    if (v0 >= v1) v0 = v1;
    if (total >= c) break;
    total++;
  }
  printf("%d\n", d);
  return 0;
}
