#include <bits/stdc++.h>
using namespace std;

int n,m,ans,t[15];
char a[15][15];
bool f,vis[15];

void dfs(int pos,int r) {
	if (pos - 1 == r) {
		bool flag[15] = {};
		for (int i = 1;i <= r;i++) 
			for (int j = 1;j <= m;j++) 
				if (a[t[i]][j] == 'o') flag[j] = 1;
		for (int i = 1;i <= m;i++) 
			if (flag[i] == 0) return ;
		ans = r,f = 1;
		return ; 
	}
	if (f == 1) return ;
	for (int i = 1;i <= n;i++) {
		if (vis[i] == 0) {
			vis[i] = 1;
			t[pos] = i;
			dfs(pos + 1,r);
			vis[i] = 0;
		}
	}
}

int main () {
	cin >> n >> m;
	for (int i = 1;i <= n;i++) 
		for (int j = 1;j <= m;j++) cin >> a[i][j];
	for (int i = 1;i <= n && f == 0;i++) dfs(1,i);
	cout << ans;
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
#define int long long
char str[20][20];
bool v[20];
int n,m;
bool pd()
{
	for(int i=1;i<=m;i++)if(v[i]==0)return 0;
	return 1;
}
signed main()
{
	scanf("%lld%lld",&n,&m);
	for(int i=1;i<=n;i++)scanf("%s",str[i]+1);
	int ans=n;
	for(int S=1;S<(1<<n);S++)
	{
		int sum=0;memset(v,0,sizeof v);
		for(int i=1;i<=n;i++)if(S&(1<<(i-1)))
		{
			sum++;
			for(int j=1;j<=m;j++)
				if(str[i][j]=='o')v[j]=1;
		}
		if(pd())ans=min(ans,sum);
	}
	printf("%lld\n",ans);
	return 0;
}