/* -*- coding: utf-8 -*-
 *
 * 3615.cc:  No.3615 Ge Gusser - yukicoder
 */

#include<cstdio>
#include<algorithm>

using namespace std;

/* constant */

const int MAX_N = 22;
const int NBITS = 1 << MAX_N;
const int MAX_M = 50;
const int BN = 17;
const int BBITS = 1 << BN;
const int BMSK = BBITS - 1;

/* typedef */

using ll = long long;

/* global variables */

ll bs[MAX_N], bbs[NBITS];
int bnums[BBITS];
double dp0[NBITS], dp1[NBITS];

/* subroutines */

int bitnum(ll b) {
  return
    bnums[b & BMSK] +
    bnums[(b >> BN) & BMSK] +
    bnums[(b >> (BN * 2)) & BMSK];
}

/* main */

int main() {
  int n, m;
  scanf("%d%d", &n, &m);
  for (int i = 0; i < n; i++) {
    char s[MAX_M + 4];
    scanf("%s", s);
    for (int j = 0; j < m; j++)
      if (s[j] == 'o') bs[i] |= (1LL << j);
  }
  //for (int i = 0; i < n; i++) printf(" %lld", bs[i]); putchar('\n');

  for (int bits = 1, msb = 1; bits < BBITS; bits++) {
    if ((msb << 1) <= bits) msb <<= 1;
    bnums[bits] = bnums[bits ^ msb] + 1;
  }

  int nbits = 1 << n;
  ll mbits = 1LL << m, mmsk = mbits - 1;
  bbs[0] = mmsk;
  for (int bits = 1, msb = 1, msi = 0; bits < nbits; bits++) {
    if ((msb << 1) <= bits) msb <<= 1, msi++;
    bbs[bits] = bbs[bits ^ msb] & bs[msi];
  }
  //for (int bits = 0; bits < nbits; bits++) printf(" %lld", bbs[bits]);
  //putchar('\n');

  dp0[0] = 1.0;
  for (int bits = 0; bits < nbits; bits++) {
    int bn = bitnum(bits);
    //printf(" dp[%d] = %lf, bn=%d, %d\n",
    //	   bits, dp[bits], bn, bitnum(bbs[bits]));
    double dd = (bitnum(bbs[bits]) != 1) ? dp0[bits] : 0.0;
    for (int i = 0, bi = 1; i < n; i++, bi <<= 1)
      if (! (bits & bi)) {
	dp0[bits | bi] += dp0[bits] / (n - bn);
	dp1[bits | bi] += (dp1[bits] + dd) / (n - bn);
      }
  }

  printf("%.10lf\n", dp1[nbits - 1]);
  
  return 0;
}