#include <bits/stdc++.h>
using namespace std;
#define pii pair<int,int>
const int nmax=4e3+2;
int mat[nmax][nmax];
bool visited[nmax][nmax];
int dist[nmax][nmax];
int n,m;
int di[]={-1,1,0,0};
int dj[]={0,0,-1,1};
bool inmat(int i,int j){
return 1<=i&&i<=n&&1<=j&&j<=m;
}
void bfs1_0(int is,int js){
deque<pii>q;
q.push_front({is,js});
visited[is][js]=1;
dist[is][js]=1;
while(!q.empty()){
pii current=q.front();
q.pop_front();
for(int k=0;k<4;k++){
int in=current.first+di[k];
int jn=current.second+dj[k];
if(inmat(in,jn)&&!visited[in][jn]&&mat[in][jn]){
visited[in][jn]=1;
if(mat[current.first][current.second]==mat[in][jn]){
q.push_front({in,jn});
dist[in][jn]=dist[current.first][current.second];
}
else{
q.push_back({in,jn});
dist[in][jn]=dist[current.first][current.second]+1;
}
}
}
}
}
void read(){
cin>>n>>m;
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++){
char a;
cin>>a;
if(a=='.')mat[i][j]=0;
if(a=='F')mat[i][j]=1;
if(a=='R')mat[i][j]=2;
}
}
}
int main(){
read();
bfs1_0(1,1);
int sol=-1;
for(int i=1;i<=n;i++){
for(int j=1;j<=m;j++)
sol=max(sol,dist[i][j]);
}
cout<<sol;
}
