#include <bits/stdc++.h>
#define pb push_back
#define F first
#define S second
#define all(v) v.begin(),v.end()
#define Gondozu ios::sync_with_stdio(false);cout.tie(NULL);cin.tie(NULL);

using namespace std;
using ll = long long;
using pi = pair<int, int>;
using vi = vector<int>;
using vpi = vector <pair<int, int>>;
using vvi = vector <vector<int>>;

const int OO = 1e9 + 5;
const int N = 26, MX_S = 1000;

double dis(int x1,int y1, int x2, int y2){
    return sqrt((x1-x2)*(x1-x2) + (y1-y2)*(y1-y2));
}

int h,w,r,s,tx,bx,by;
char g[N][N];

bool valid(int x, int y){
    return x >= 0 && x < h && y >= 0 && y < w && g[x][y] != '.';
}

double dp[N][N][MX_S];
bool vis[N][N][MX_S];

double slv(int x, int y, int rem){
    if(rem < 0)
        return OO;
    if(x == tx)
        return 0;

    double &ret = dp[x][y][rem];
    if(vis[x][y][rem])
        return ret;
    vis[x][y][rem] = true;
    ret = OO;

    for (int dx = -3; dx <= 3; ++dx) {
        for (int dy = -3; dy <= 3; ++dy) {
            int nx = x + dx, ny = y + dy;
            if(valid(nx, ny) && dis(x,y,nx,ny) <= r)
                ret = min(ret, dis(x,y,nx,ny) + slv(nx,ny,rem - (g[nx][ny]-'0')));
        }
    }

    return ret;
}

void TC()
{
    cin >> h >> w >> r >> s;

    tx = -1;
    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            cin >> g[i][j];
            if(g[i][j] == '.')continue;
            if(!~tx) tx = i;
            bx = i, by = j;
        }
    }

    double ans = slv(bx,by,min(MX_S-1, s) - (g[bx][by]-'0'));
    if(ans == OO) cout << "impossible";
    else cout << fixed << setprecision(8)   << ans;
}

int32_t main() {
#ifndef ONLINE_JUDGE
    freopen("input.in", "r", stdin); freopen("output.out", "w", stdout);
#endif
    Gondozu
    int t = 1;
//    cin >> t;
    while (t--) {
        TC();
        cout << '\n';
    }
    return 0;
}