#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r){return l+rng()%(r-l+1);}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0);
#define forinc(a,b,c) for(int a=b,_c=c;a<=_c;++a)
#define fordec(a,b,c) for(int a=b,_c=c;a>=_c;--a)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f, x) memset(f, x, sizeof(f))
#define bit(x,i) ((x>>(i-1))&1)
#define on(x,i) (x|(1ll<<(i-1)))
#define off(x,i) (x&~(1<<(i-1)))
#define gg exit(0);
struct pack{
int u,v,c;
};
const int N=5010,
INF=1e9;
int m,n;
int a[N][N];
vector<pack> e;
vector<int> ad[N];
int it,s,t;
int tr[N],id[N][N],st[N][N],ed[N][N];
void add(int u,int v){
ad[u].pb(e.size()); e.pb({u,v,1});
ad[v].pb(e.size()); e.pb({v,u,0});
}
int bfs(){
reset(tr,-1);
queue<int> q; q.push(s); tr[s]=-2;
while(q.size()){
int u=q.front(); q.pop();
if(u==t) return 1;
forv(i,ad[u]){
if(tr[e[i].v]==-1 && e[i].c)
tr[e[i].v]=i, q.push(e[i].v);
}
}
return 0;
}
main(){
#define task "rgw"
//freopen(task".inp","r",stdin);
//freopen(task".out","w",stdout);
fasty;
cin>>m>>n;
forinc(i,1,m){
string s; cin>>s;
forinc(j,1,n){
a[i][j]=s[j-1]=='R' ? 1 : s[j-1]=='G' ? 2 : 3;
}
}
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==1) id[i][j]=++it;
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==3) id[i][j]=++it;
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==2) st[i][j]=++it, ed[i][j]=++it, add(it-1,it);
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==2){
if(1<i && i<m && a[i][j]+a[i-1][j]+a[i+1][j]==6 && a[i-1][j]==1)
add(id[i-1][j],st[i][j]), add(ed[i][j],id[i+1][j]);
if(1<j && j<n && a[i][j]+a[i][j-1]+a[i][j+1]==6 && a[i][j-1]==1)
add(id[i][j-1],st[i][j]), add(ed[i][j],id[i][j+1]);
}
s=++it, t=++it;
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==1) add(s,id[i][j]);
forinc(i,1,m) forinc(j,1,n) if(a[i][j]==3) add(id[i][j],t);
int flow=0;
while(bfs()){
int d=INF;
int u=t;
vector<int> path;
while(u!=s){
d=min(d,e[tr[u]].c);
path.push_back(tr[u]);
u=e[tr[u]].u;
}
forv(i,path) e[i].c-=d, e[i^1].c+=d;
flow+=d;
}
cout<<flow;
}
