//#pragma GCC optimize("Ofast,unroll-loops")
#include <bits/stdc++.h>
using namespace std;
typedef long long llo;
#define mp make_pair
#define pb push_back
#define a first
#define b second
//define endl '\n'
llo n,m;
llo it[1001][11];
llo dp[1001][2001];
llo val[50][50][10];//first half is i has k bits set in (j^second half of number)
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin>>n>>m;
for(llo i=0;i<n;i++){
string s;
cin>>s;
for(llo j=0;j<m;j++){
if(s[j]=='#'){
it[i][j]=1;
}
else{
it[i][j]=0;
}
}
}
for(llo j=0;j<(1<<m);j++){
dp[0][j]=1e9;
}
dp[0][0]=0;
for(llo i=1;i<=n;i++){
//cout<<i<<endl;
for(llo j=0;j<32;j++){
for(llo k=0;k<32;k++){
for(llo p=0;p<=5;p++){
val[j][k][p]=1e9;
}
}
}
for(llo j=0;j<(1<<m);j++){
for(llo k=0;k<32;k++){
llo kk=__builtin_popcount(k&(((j-(j&31)))/32));
val[j&31][k][kk]=min(val[j&31][k][kk],dp[i-1][j]);
}
}
for(llo j=0;j<(1<<m);j++){
dp[i][j]=1e9;
llo stt=0;
for(llo k=0;k<m;k++){
if(it[i-1][k]==0 and (j&(1<<k))){
stt=1;
break;
}
}
if(stt){
continue;
}
llo ba=-2;
llo cot=0;
for(llo k=0;k<m;k++){
if(it[i-1][k]==1){
if((1<<k)&j){
}
else{
if(k>ba+1){
cot++;
}
ba=k;
}
}
}
/* for(llo k=0;k<(1<<m);k++){
dp[i][j]=min(dp[i][j],dp[i-1][k]+cot+__builtin_popcount(j)-__builtin_popcount(j&k));
}*/
for(llo k=0;k<32;k++){
for(llo kk=0;kk<=5;kk++){
llo co=__builtin_popcount(j)-kk-__builtin_popcount((j&31)&k);
//if(val[k][(j-(j&31))/32][kk]+co+cot==2){
//cout<<i<<":"<<j<<":"<<k<<":"<<kk<<endl;
//cout<<val[k][(j-(j&31))/32][kk]<<"//"<<co<<"//"<<cot<<endl;
//}
dp[i][j]=min(dp[i][j],val[k][(j-(j&31))/32][kk]+co+cot);
}
}
}
}
//cout<<33<<endl;
// cout<<n<<":"<<m<<endl;
//return 0;
llo ans=n*m;
/*for(llo i=0;i<(1<<m);i++){
cout<<dp[1][i]<<":";
}
cout<<endl;*/
for(llo i=0;i<(1<<m);i++){
//cout<<i<<":"<<dp[n][i]<<endl;
// continue;
ans=min(ans,dp[n][i]);
/* if(dp[n][i]==2){
cout<<i<<":"<<endl;
}*/
}
cout<<ans<<endl;
return 0;
}
