pokemon evolution

#include <bits/stdc++.h>

using namespace std;
typedef unsigned long long ull;
constexpr ull nmax=6;
/// M + (N - A) * Y >= A * X
ull n,m,x,y,evo=0;

ull cautbin(ull st,ull dr){
    ull a = (st+dr+1)/2;
    if(st>dr) return -1;
    if(st==dr){
        return a;
    }

    if( m + (n-a)*y >= a*x ) return cautbin(a,dr);
    else return cautbin(st,a-1);
}

int main() {

    cin>>n>>m>>x>>y;
    cout<<cautbin(0,n);

    return 0;
}