#include <bits/stdc++.h>

#define endl            '\n'
#define pb             push_back
#define fill(x, y)     memset(x, y, sizeof(x))
#define all(x)         (x).begin(), (x).end()
#define debug(x)       { cerr << #x << " = " << x << endl; }
#define IO	       { ios_base::sync_with_stdio(false); cin.tie(0); }
#define read(x)	       freopen(x, "r", stdin)
#define write(x)       freopen(x, "w", stdout)

using namespace std;

typedef long long      ll;
typedef pair<int, int> ii;
typedef vector<int>    vi;

const double inf = 1e18;
const double pi = acos(-1);

void solve() {
    double n, m; cin>> n>> m;
    double a, b; cin>> a>> b;
    double k = (a*n-b*m)/(n-m);
    (k<0 || k>100)? cout<< "impossible"<< endl : cout<< k<< endl;

}

int main() {
    IO;
    cout << fixed << setprecision(10);
    solve();
}