package chelper;

import io.InputReader;
import io.OutputWriter;
import misc.GCJSolution;


public class TaskA extends GCJSolution {

  private int n;
  private int m;
  private int cutsX;
  private int cutsY;
  private boolean[][] map;
  private int[][] ps;

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  int sum(int x0, int y0, int x1, int y1) {
    int res = ps[x1][y1];

    if (x0 > 0) {
      res -= ps[x0 - 1][y1];
    }
    if (y0 > 0) {
      res -= ps[x1][y0 - 1];
    }
    if (x0 > 0 && y0 > 0) {
      res += ps[x0 - 1][y0 - 1];
    }

    return res;
  }

  @Override
  public void solve(int testNumber) {
    n = in.nextInt();
    m = in.nextInt();

    cutsX = in.nextInt();
    cutsY = in.nextInt();

    map = new boolean[n][m];
    for (int i = 0; i < n; i++) {
      String s = in.nextString();
      for (int j = 0; j < m; j++) {
        map[i][j] = s.charAt(j) == '@';
      }
    }

    ps = new int[n][m];

    for (int i = 0; i < n; i++) {
      for (int j = 0; j < m; j++) {
        if (i > 0) {
          ps[i][j] += ps[i - 1][j];
        }
        if (j > 0) {
          ps[i][j] += ps[i][j - 1];
        }
        if (i > 0 && j > 0) {
          ps[i][j] -= ps[i - 1][j - 1];
        }

        if (map[i][j]) {
          ps[i][j]++;
        }
      }
    }

    int[] cx0 = new int[cutsX + 1];
    int[] cx1 = new int[cutsX + 1];
    int[] cy0 = new int[cutsY + 1];
    int[] cy1 = new int[cutsY + 1];

    int total = sum(0, 0, n - 1, m - 1);
    int cutC = (cutsX + 1) * (cutsY + 1);

    if (total % cutC != 0) {
      out.println("IMPOSSIBLE");
      return;
    }

    int perC = total / cutC;

    int cutStartX = 0;
    int cutStartY = 0;

    for (int i = 0; i < cutsX + 1; i++) {
      boolean ok = false;

      for (int cutEndX = cutStartX; cutEndX < n; cutEndX++) {
        int s = sum(cutStartX, 0, cutEndX, m - 1);

        if (s == perC * (cutsY + 1)) {
          cx0[i] = cutStartX;
          cx1[i] = cutEndX;

          cutStartX = cutEndX + 1;
          ok = true;
          break;
        }
      }

      if (!ok) {
        out.println("IMPOSSIBLE");
        return;
      }
    }

    for (int i = 0; i < cutsY + 1; i++) {
      boolean ok = false;

      for (int cutEndY = cutStartY; cutEndY < m; cutEndY++) {
        int s = sum(0, cutStartY, n - 1, cutEndY);

        if (s == perC * (cutsX + 1)) {
          cy0[i] = cutStartY;
          cy1[i] = cutEndY;

          cutStartY = cutEndY + 1;
          ok = true;
          break;
        }
      }

      if (!ok) {
        out.println("IMPOSSIBLE");
        return;
      }
    }

    for (int i = 0; i < cutsX + 1; i++) {
      for (int j = 0; j < cutsY + 1; j++) {
        int s = sum(cx0[i], cy0[j], cx1[i], cy1[j]);
        if (s != perC) {
          out.println("IMPOSSIBLE");
          return;
        }
      }
    }

    out.println("POSSIBLE");
  }
}