#include <bits/stdc++.h>
#define ll int
#define MAXN 4001
using namespace std;
char c[MAXN][MAXN];
ll dist[MAXN][MAXN];
int main()
{
ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
ll h,w;
cin >> h >> w;
for (ll i=1;i<=h;i++)
{
for (ll j=1;j<=w;j++)
cin >> c[i][j];
}
deque<pair<ll,ll> > q;
q.push_back({1,1});
dist[1][1]=1;
ll ans=0;
while (!q.empty())
{
ll i=q.front().first,j=q.front().second;
q.pop_front();
ans=max(ans,dist[i][j]);
vector<pair<ll,ll> > same,diff;
for (ll x=-1;x<2;x++)
{
for (ll y=-1;y<2;y++)
{
if (abs(x)+abs(y)==1 && (c[i+x][j+y]=='R' || c[i+x][j+y]=='F') && !dist[i+x][j+y])
{
dist[i+x][j+y]=dist[i][j]+(c[i+x][j+y]!=c[i][j]);
if (c[i+x][j+y]!=c[i][j])
diff.push_back({i+x,j+y});
else
same.push_back({i+x,j+y});
}
}
}
for (auto x : same)
q.push_front(x);
for (auto x : diff)
q.push_back(x);
}
cout << ans;
return 0;
}
