Contest Struggles

//计算出来的剩下平均值0到100就输出，否则impossible
#include 
using namespace std;
double n, k, d, s;
void solve(){
	cin >> n >> k >> d >> s;
	double avg = (n * d - k * s) / (n - k);
	if (avg < 0 || avg>100) {
		printf("impossible");
		return;
	}
	printf("%.7f", avg);
}
int main(){
	solve();
	return 0;
}