#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <utility>
#include <algorithm>
#include <cmath>
#include <climits>
#include <iomanip>
#include <queue>
#include <stack>
#include <ctype.h>

using namespace std;

typedef long long ll;

const int INF = (1<<30)-1;
const ll LINF = 1e18;

#define rep(i, n) for (int i = 0; i < n; i++)

template<class T> 
bool chmax(T &a, T b) {if (a < b) {a = b;return true;}else return false;}

template<class T> 
bool chmin(T &a, T b) {if (a > b) {a = b;return true;}else return false;}

int main() {
    int n, m, k;    cin >> n >> m >> k;
    vector<int> a(n+1, 0);
    rep(i, k) {
        int x;  cin >> x;
        a[x] = 1;
    }
    for (int i = 0; i < n+1; i++) {
        int j = i;
        while (j < n+1 && a[j] == 1)
            j++;
        if (j - i >= m) {
            cout << -1 << endl;
            return 0;
        }
        i = j;
    }
    vector<double> b(n+m+10), c(n+m+10), sb(n+m+10, 0), sc(n+m+10, 0);
    for (int i = n-1; i >= 0; i--) {
        if (a[i]) {
            b[i] = 1;
            c[i] = 0;
        }
        else {
            b[i] = (sb[i+1] - sb[i+m+1]) / m;
            c[i] = (sc[i+1] - sc[i+m+1]) / m + 1;
        }
        sb[i] = b[i] + sb[i+1];
        sc[i] = c[i] + sc[i+1];
    }
    cout << fixed << std::setprecision(15) << c[0] / (1 - b[0]) << endl;
    return 0;
}

//小数点精度
//cout << fixed << std::setprecision(15) << y << endl;
#pragma GCC optimize ("Ofast")
#include<bits/stdc++.h>
using namespace std;
inline int my_getchar_unlocked(){
  static char buf[1048576];
  static int s = 1048576;
  static int e = 1048576;
  if(s == e && e == 1048576){
    e = fread_unlocked(buf, 1, 1048576, stdin);
    s = 0;
  }
  if(s == e){
    return EOF;
  }
  return buf[s++];
}
inline void rd(int &x){
  int k;
  int m=0;
  x=0;
  for(;;){
    k = my_getchar_unlocked();
    if(k=='-'){
      m=1;
      break;
    }
    if('0'<=k&&k<='9'){
      x=k-'0';
      break;
    }
  }
  for(;;){
    k = my_getchar_unlocked();
    if(k<'0'||k>'9'){
      break;
    }
    x=x*10+k-'0';
  }
  if(m){
    x=-x;
  }
}
inline void rd(char &c){
  int i;
  for(;;){
    i = my_getchar_unlocked();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c = i;
}
inline int rd(char c[]){
  int i;
  int sz = 0;
  for(;;){
    i = my_getchar_unlocked();
    if(i!=' '&&i!='\n'&&i!='\r'&&i!='\t'&&i!=EOF){
      break;
    }
  }
  c[sz++] = i;
  for(;;){
    i = my_getchar_unlocked();
    if(i==' '||i=='\n'||i=='\r'||i=='\t'||i==EOF){
      break;
    }
    c[sz++] = i;
  }
  c[sz]='\0';
  return sz;
}
struct MY_WRITER{
  char buf[1048576];
  int s;
  int e;
  MY_WRITER(){
    s = 0;
    e = 1048576;
  }
  ~MY_WRITER(){
    if(s){
      fwrite_unlocked(buf, 1, s, stdout);
    }
  }
}
;
MY_WRITER MY_WRITER_VAR;
void my_putchar_unlocked(int a){
  if(MY_WRITER_VAR.s == MY_WRITER_VAR.e){
    fwrite_unlocked(MY_WRITER_VAR.buf, 1, MY_WRITER_VAR.s, stdout);
    MY_WRITER_VAR.s = 0;
  }
  MY_WRITER_VAR.buf[MY_WRITER_VAR.s++] = a;
}
inline void wt_L(char a){
  my_putchar_unlocked(a);
}
inline void wt_L(int x){
  int s=0;
  int m=0;
  char f[10];
  if(x<0){
    m=1;
    x=-x;
  }
  while(x){
    f[s++]=x%10;
    x/=10;
  }
  if(!s){
    f[s++]=0;
  }
  if(m){
    my_putchar_unlocked('-');
  }
  while(s--){
    my_putchar_unlocked(f[s]+'0');
  }
}
int X;
int Y;
char S[10][12];
int res;
int main(){
  int i;
  int j;
  int k;
  rd(X);
  rd(Y);
  {
    int Lj4PdHRW;
    for(Lj4PdHRW=(0);Lj4PdHRW<(X);Lj4PdHRW++){
      rd(S[Lj4PdHRW]);
    }
  }
  for(i=(1);i<(X);i++){
    for(j=(1);j<(Y);j++){
      k = 0;
      if(S[i-1][j-1] == '#'){
        k++;
      }
      if(S[i-1][j  ] == '#'){
        k++;
      }
      if(S[i  ][j-1] == '#'){
        k++;
      }
      if(S[i  ][j  ] == '#'){
        k++;
      }
      res += k % 2;
    }
  }
  wt_L(res);
  wt_L('\n');
  return 0;
}
// cLay version 20210103-1 [bug fixed 2]

// --- original code ---
// int X, Y;
// char S[10][12];
// int res;
// {
//   int i, j, k;
//   rd(X,Y,S(X));
//   rep(i,1,X) rep(j,1,Y){
//     k = 0;
//     if(S[i-1][j-1] == '#') k++;
//     if(S[i-1][j  ] == '#') k++;
//     if(S[i  ][j-1] == '#') k++;
//     if(S[i  ][j  ] == '#') k++;
//     res += k % 2;
//   }
//   wt(res);
// }
