#include <bits/stdc++.h>

#define pb push_back
#define F first
#define S second
#define MP make_pair
#define all(x) x.begin(),x.end()
#define Hagry 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>>;
using ld = long double;

const ld OO = 1e15;
const int N = 2e5 + 5;
const int H = 25 + 4;
const int maxS = 6e3 + 5;
int h, w, r, s;

char grid[H][H];
ld dp[H][H][maxS];
int vis[H][H][maxS], vid;
vector<pair<pi, int>> adj[H][H];
pi start, target;
ld solve(int i, int j, int remS) {
    int cost = (grid[i][j] - '0');
    remS -= cost;
    if(remS < 0)return OO;

    if(MP(i, j) == target)
        return 0;

    ld &ret = dp[i][j][remS];
    if(vis[i][j][remS] == vid)
        return ret;
    vis[i][j][remS] = vid;

    ret = OO;
    for(auto e:adj[i][j]){
        ret = min(ret, sqrt((double)e.S) + solve(e.F.F, e.F.S, remS));
    }
    return ret;
}

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

    target = {-1, -1};
    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            cin >> grid[i][j];
            if (grid[i][j] != '.') {
                if (target == MP(-1, -1))
                    target = MP(i, j);
                start = MP(i, j);
            }
        }
    }

    for (int i = 0; i < h; ++i) {
        for (int j = 0; j < w; ++j) {
            if (grid[i][j] == '.')continue;
            for (int d1 = -3; d1 <= 3; ++d1) {
                for (int d2 = -3; d2 <= 3; ++d2) {
                    int newI = d1 + i;
                    int newJ = d2 + j;
                    if (newI < 0 || newI >= h ||
                        newJ < 0 || newJ >= w ||
                        grid[newI][newJ] == '.')
                        continue;
                    int distSq = abs(d1) * abs(d1) + abs(d2) * abs(d2);
                    if (r * r < distSq)continue;
                    adj[i][j].push_back(MP(MP(newI, newJ), distSq));
                }
            }
        }
    }

    s = min(s, maxS);
    ld ans = solve(start.F, start.S, s);
    if (ans >= OO)
        cout << "impossible\n";
    else
        cout << fixed << setprecision(9) << ans;
}
int32_t main() {
#ifndef ONLINE_JUDGE
    freopen("input.in", "r", stdin);
    freopen("output.out", "w", stdout);
#endif
    Hagry
    int t = 1;
//    cin >> t;
    while (t--) {
        TC();
        cout << '\n';
    }
    return 0;
}