#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll ILL=2167167167167167167;
const int INF=2100000000;
const ll mod=998244353;
#define rep(i,a) for (ll i=0;i<a;i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}
template<class T> void vec_out(vector<T> &p){for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}
template<class T> T min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T sum(vector<T> &a){assert(!a.empty());T ans=a[0]-a[0];for(auto &x:a) ans+=x;return ans;}

//a*x+b*y=gcd(a,b)となるx,yにする　返り値gcd(a,b)
ll Euclid(ll a,ll b,ll &x,ll &y){
    if(b==0){
        x=1,y=0;
        return a;
    }
    ll d=Euclid(b,a%b,y,x);
    y-=a/b*x;
    return d;
}

// val1=(a,p) val2=(b,q)
// return (c,r)
// c: c%p==a && c%q==b
// r: r=lcm(a,b)
// need: a%gcd(p,q)==b%gcd(p,q)
// use: Euclid
std::pair<long long,long long> ctr_sub(std::pair<long long,long long> val1,std::pair<long long,long long> val2){
	long long a,b,p,q;
	long long X,Y,G,ans_val,ans_mod;
	tie(a,p)=val1;
	tie(b,q)=val2;
	G=Euclid(p,q,X,Y);
	if((b-a)%G!=0) return {-1,-1};
	ans_mod=p*(q/G);
	ans_val=(p*((X*((b-a)/G))%q))%ans_mod+a;
	return {(ans_val%ans_mod+ans_mod)%ans_mod,ans_mod};
}

void solve();
// oddloop
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int t=1;
	//cin>>t;
	rep(i,t) solve();
}

void solve(){
	ll a,b,c,d;
	cin>>a>>b>>c>>d;
	b=(b%a+a)%a;
	d=(d%c+c)%c;
	ll e=__gcd(a,c);
	if(b%e!=d%e){
		cout<<"NaN\n";
	}else{
		cout<<ctr_sub({b,a},{d,c}).first<<"\n";
	}
}