#include <bits/stdc++.h>
typedef long long LL;
#define SORT(c) sort((c).begin(),(c).end())

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)
using namespace std;
int cost[512][512][2];
int h,w;
vector<string> fi;

int main(void)
{
  cin >> h >> w;
  REP(i,h) REP(j,w) REP(k,2) cost[i][j][k]=-1048576;
  int dx[12]={1,1,-1,-1,1,1,2,2,-1,-1,-2,-2};
  int dy[12]={1,-1,-1,1,2,-2,1,-1,2,-2,1,-1};
  fi.resize(h);
  REP(i,h) cin >> fi[i];
  priority_queue<tuple<int,int,int,int> > dijk;
  REP(i,h) REP(j,w) if(fi[i][j]=='S') dijk.push(make_tuple(0,i,j,0));
  // negcos,x,y,type;
  while(!dijk.empty()){
    int negc,x,y,t;
    tie(negc,x,y,t)=dijk.top();
    dijk.pop();
    if(x<0||h<=x||y<0||w<=y) continue;
    if(cost[x][y][t]>=negc) continue;
    cost[x][y][t]=negc;
    if(fi[x][y]=='R') t=1-t;
    if(t) REP(i,4) dijk.push(make_tuple(negc-1,x+dx[i],y+dy[i],t));
    else REP(i,8) dijk.push(make_tuple(negc-1,x+dx[i+4],y+dy[i+4],t));
  }
  int answer;
  REP(i,h) REP(j,w) if(fi[i][j]=='G') answer=-max(cost[i][j][0],cost[i][j][1]);
  if(answer>1048000) cout << -1 << endl;
  else cout << answer << endl;
  return 0;

}