// created:  2025-03-19 19:19:19
// code->debug 2025-03-19 20:21:31
#include<cstdio>
#include<cctype>
#include<cstring>
#include<algorithm>
#define F(i,l,r) for(int i=(l),i##_end=(r);i<i##_end;++i)
#define I128 //||is_same<T,__int128_t>::value||is_same<T,__uint128_t>::value
using namespace std;
template<typename T>enable_if_t<is_integral<T>::value I128,void> readmain(T &x)
{
	bool neg=false;int c=getchar();
	for(;!isdigit(c);c=getchar())if(c=='-')neg=true;
	for(x=0;isdigit(c);c=getchar())x=(T)(10*x+(c-'0'));
	if(neg)x=-x;
}
template<typename T>T& read(T &x){readmain(x);return x;}
template<typename T,typename ...Tr>void read(T &x,Tr&... r){readmain(x);read(r...);}
constexpr int N=20,M=1e5+5,INF=0x3fffffff;
int gcd(int a,int b){return !b?a:gcd(b,a%b);}
struct frac
{
	int a,b;
	friend bool operator<(const frac &u,const frac &v){return u.a*v.b<v.a*u.b;}
};
int num,den;
struct segtree
{
	static constexpr int N=(1<<18)+5;
	struct cont
	{
		int len,a,p,s,m;
		friend cont operator+(const cont &u,const cont &v)
		{
			return cont{u.len+v.len,u.a&v.a,u.a?v.p+u.len:u.p,v.a?u.s+v.len:v.s,min({u.m,u.len+v.m,u.s+v.p>=num?u.len-u.s:INF})};
		}
		void modify(const cont &u,const cont &v)
		{
			a=u.a&v.a;
			p=(u.a&(v.p+u.len))|(~u.a&u.p);
			s=(v.a&(u.s+v.len))|(~v.a&v.s);
			m=min(u.m,u.s+v.p>=num?u.len-u.s:u.len+v.m);
		}
	};
	struct node
	{
		int mv,tv;
		cont v0,v1;
		void get0(){v1=v0;}
		friend node operator+(node a,node b)
		{
			b.mv+=a.tv;
			if(a.mv>b.mv)a.get0();
			if(a.mv<b.mv)b.get0();
			return node{min(a.mv,b.mv),a.tv+b.tv,a.v0+b.v0,a.v1+b.v1};
		}
		void modify(const node &a,const node &b)
		{
			int bv=b.mv+a.tv;
			tv=a.tv+b.tv;
			if(a.mv>bv)v1.modify(a.v0,b.v1),mv=bv;
			else if(a.mv<bv)v1.modify(a.v1,b.v0),mv=a.mv;
			else v1.modify(a.v1,b.v1),mv=a.mv;
		}
	}s[N];
	int n;
	void build(int n_,int *c,char *ma)
	{
		for(n=1;n<n_;n<<=1);
		node *leaf=s+n;
		cont c0{den,0,0,0,INF},c1{den,-1,den,den,den>=num?0:INF};
		F(i,0,n)leaf[i]={i<n_?c[i]:INF,0,i<n_&&ma[i]=='.'?c1:c0,c0};
		for(int i=n;--i;)s[i]=s[2*i]+s[2*i+1];
	}
	void update(int p,int mv,int tv,cont v)
	{
		p+=n;
		s[p].mv=mv;s[p].tv=tv;
		if(s[p].v0.a)s[p].v1=v;
		while(p>>=1)s[p].modify(s[2*p],s[2*p+1]);
	}
	int query()
	{
		if(s[1].mv==1)return s[1].v1.m;
		else return s[1].v0.m;
	}
}s[N];
int n,m,c[M],b[N*M];
char a[N][M];
void update(int re,int x,int v)
{
	if(x==m*den)return;
	b[x]+=v;x/=den;
	int mv=INF,tv=0;
	segtree::cont d{den,0,0,0,INF};
	static int ss[M];
	F(i,0,den)mv=min(mv,ss[i]=tv+=b[x*den+i]);
	int y=1;
	for(int i=den;~--i;)
	{
		if(ss[i]>mv)++d.p,d.s+=y;
		else d.p=0,y=0;
		if(d.p>=num)d.m=i;
	}
	mv+=c[x];
	while(re)
	{
		int i=__builtin_ctz(re);re^=1<<i;
		s[i].update(x,mv,tv,d);
	}
}
bool dfs(int rem)
{
	if(!rem)return true;
	pair<int,int> mn[2]={{INF,-1},{INF,-1}};
	for(int j=rem;j;)
	{
		int i=__builtin_ctz(j);j^=1<<i;
		pair<int,int> cur={s[i].query(),i};
		if(cur.first>=m*den)return false;
		if(cur<mn[0])mn[1]=mn[0],mn[0]=cur;
		else mn[1]=min(mn[1],cur);
	}
	for(pair<int,int> ii:mn)if(~ii.second&&ii.first<mn[0].first+num)
	{
		int i=ii.second,l=ii.first,re=rem^1<<i;
		update(re,l,-1);update(re,l+num,1);
		if(dfs(re))return true;
		update(re,l,1);update(re,l+num,-1);
	}
	return false;
}
int ians=-1;
bool check(frac val)
{
	if(~ians)
	{
		if(!(frac{ians,1}<val))return true;
		if(!(val<frac{ians+1,1}))return false;
	}
	num=val.a;den=val.b;
	memset(b,0,sizeof(int)*m*den);
	F(i,0,n)s[i].build(m,c,a[i]);
	return dfs((1<<n)-1);
}
frac fr[N*M];
int frc;
void genfr(int l,int r)
{
	if(r-l==1)
	{
		int ma=0;
		F(i,0,l)ma|=(gcd(i,l)==1)<<i;
		F(i,0,m+1)if(ma>>(i%l)&1)fr[frc++]={i,l};
		return;
	}
	int mid=(l+r)>>1;
	int fl=frc;
	genfr(l,mid);
	int fmid=frc;
	genfr(mid,r);
	inplace_merge(fr+fl,fr+fmid,fr+frc);
}
int main()
{
	read(n,m);
	F(i,0,n)scanf("%s",a[i]);
	F(i,0,n)F(j,0,m)c[j]+=a[i][j]=='.';
	F(i,0,m)if(!c[i])return puts("-1"),0;
	genfr(1,n+1);
	int il=0,ir=(m>>1)+1;
	while(ir-il>1)
	{
		int mid=(il+ir)>>1;
		if(check({mid,1}))il=mid;
		else ir=mid;
	}
	ians=il;
	int l=0,r=frc;
	while(r-l>1)
	{
		int mid=(l+r)>>1;
		if(check(fr[mid]))l=mid;
		else r=mid;
	}
	printf("%d/%d\n",fr[l].a,fr[l].b);
	return 0;
}