#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pii pair<int, int>
#define INF 200000001
#define EPS 1e-9

typedef long long ll;
typedef complex<double> cd;

const ll mod = 1e9+7;
const double pi = acos(-1);

int n,i,j,k,t;

ll inverse(ll val, ll mod){
	if(val == 1)
		return 1;
	ll ans = (-mod/val*inverse(mod%val, mod))%mod;
	if(ans<0)
		ans += mod;
	return ans;
}

int main(){
	ll n,m,k,a;
	while(scanf("%lld %lld %lld %lld", &n, &m, &k, &a), n||m||k||a){
		ll b = a+k-n;
		if(a == 0 || m == 0){
			if(a==m){
				if(b)
					puts("Impossible");
				else
					printf("%lld\n", n);
			}
			else if(a==0){
				if(k>=n&&(k-n)%m==0)
					printf("%lld\n", k);
				else
					puts("Impossible");
			}
			else{
				if(k+a<=n&&(n-k-a)%a==0)
					printf("%lld\n", n);
				else
					puts("Impossible");
			}
		}
		else if(b % __gcd(a, m)){
			puts("Impossible");
		}
		else{
			ll e=0,f;
			if(b < 0){
				b = - b;
				ll tmp = b / a + (b%a>0);
				e += tmp;
			}
			ll d = b%m;
			bool ada = true;
			if(d > 0){
				f = __gcd(d, a);
				d /= f;
				if(__gcd(a/f, m) == 1){
					ll g = inverse((a/f)%m, m);
					ll wew = (-d*g)%m;
					e += (wew + m)%m;
				}
				else
					ada = false;
			}
			if(ada)
				printf("%lld\n", e * a + a + k);
			else
				puts("Impossible");
		}
	}
	return 0;
}