#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int c, v0, v1, a, l, ctr = 0, pg = 0, xx;
  scanf("%lld", &c);
  scanf("%lld", &v0);
  scanf("%lld", &v1);
  scanf("%lld", &a);
  scanf("%lld", &l);
  while (1) {
    xx = v0 + ctr * a;
    ctr++;
    if (xx >= v1) {
      xx = v1;
    }
    if (ctr == 1)
      pg += xx;
    else
      pg += (xx - l);
    if (pg >= c) {
      break;
    }
  }
  cout << ctr << endl;
  return 0;
}
