# include <bits/stdc++.h>
typedef long long ll;
typedef long double ld;
using namespace std;

int main() {

  ll a, b, c, d;
  cin >> a >> b >> c >> d;
  if (b/c >= d) {
    cout << -1 << '\n';
  } else {
    ll y = (d*c)-b;
    ll x = (a/y);
    if (((b*x)+a) <= (d * x * c)) {
      cout << x << '\n';
    } else {
      cout << x + 1 << '\n';
    }
  }

  return 0;
}
#include<bits/stdc++.h>
using namespace std;
template <typename T>
void read(T &x)
{
	T f = 1; x = 0;
	char ch = getchar();
	for (; !isdigit(ch); ch = getchar()) if (ch == '-') f = -1;
	for (; isdigit(ch); ch = getchar()) x = x * 10 + ch - '0';
	x *= f;
}
template <typename T>
void write(T x)
{
	if (x < 0)
	{
		putchar('-');
		x = -x;
	}
	if (x > 9) write(x / 10);
	putchar(char(x % 10 + 48));
}
template <typename T>
void writeln(T x, char ch = '\n')
{
	write(x);
	putchar(ch);
}
int n, m, k;
int main()
{
	read(n); read(m); read(k);
	cout << (n - m + k) << endl;
	return 0;
}
