#include<bits/stdc++.h>

using namespace std;

const int N = ;

int n, r, q, tot;

int id[N][N];

double S;

struct matrix {

    double a[N][N];

    matrix() { for(int i = ; i <= tot; ++i) for(int j = ; j <= tot; ++j) a[i][j] = 0.0; }

    matrix friend operator * (const matrix &a, const matrix &b) {

        matrix ret;

        for(int k = ; k <= tot; ++k)

            for(int i = ; i <= tot; ++i) if(a.a[i][k] >= 1e-)

                for(int j = ; j <= tot; ++j)

                    ret.a[i][j] += a.a[i][k] * b.a[k][j];

        return ret;

    }

    void set() {

        for(int i = ; i <= tot; ++i) a[i][i] = 1.0;

    }

};

double calc(double p)

{

    matrix A, B;

    A.set();

    /*

    第i轮j条命这一次得k分

    dp[i][j][k]=(dp[i+1][j + 1][k + 1] + k) * p

    dp[i][j][k] += dp[i + 1][j - 1][1] * (1.0 - p)

    矩阵乘法

    */

    B.a[tot][tot] = 1.0;

    for(int i = ; i <= q; ++i)

        for(int j = ; j <= r; ++j)

        {

            if(i > ) B.a[id[i - ][]][id[i][j]] = 1.0 - p;

            B.a[tot][id[i][j]] = p * (double)j;

            if(i < q && j < r) B.a[id[i + ][j + ]][id[i][j]] = p;

            else if(i < q) B.a[id[i + ][j]][id[i][j]] = p;

            else if(j < r) B.a[id[i][j + ]][id[i][j]] = p;

            else B.a[id[i][j]][id[i][j]] = p;

        }

//  for(int j = 0; j <= q; ++j)

//      for(int k = 1; k <= r; ++k) B.a[tot][id[j][k]] = p * (double)k;

//  for(int j = 0; j <= q; ++j)

//      for(int k = 1; k <= r; ++k) B.a[id[min(j + 1, q)][min(k + 1, r)]][id[j][k]] = p;

//  for(int j = 2; j <= q; ++j)

//      for(int k = 1; k <= r; ++k) B.a[id[j - 1][1]][id[j][k]] = 1.0 - p;

    for(int i = n; i; i >>= , B = B * B) if(i & ) A = A * B;

//  double ret = 0.0;

//  for(int i = 0; i <= tot; ++i)

//  {

//      for(int j = 0; j <= tot; ++j) printf("%.6f ", A.a[i][j]);

//      puts("");

//  }

//  for(int i = 0; i < tot; ++i) ret += A.a[tot][i];

//  printf("p = %.6f ret = %.6f\n", p, ret);

    return A.a[tot][id[q][]];

}

int main()

{

    scanf("%d%d%d%lf", &n, &r, &q, &S);

    for(int i = ; i <= q; ++i)

        for(int j = ; j <= r; ++j) id[i][j] = tot++;

//  printf("tot = %d\n", tot);

    double l = 0.0, r = 1.0, ans = -1.0;

    while(r - l > 1e-)

    {

        double mid = (l + r) / 2.0;

        if(calc(mid) > S) r = ans = mid;

        else l = mid;

    }

    if(ans == -1.0) puts("Impossible.");

    else printf("%.6f\n", ans);

    return ;

}