#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1010;
char mtr[MAXN][MAXN];
int main() {
  int n, m;
  cin >> n >> m;
  set<int> s;
  bool ok = true;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= m; j++) {
      cin >> mtr[i][j];
    }
  }
  for (int i = 1; i <= n; i++) {
    int gnomo, doce;
    for (int j = 1; j <= m; j++) {
      if (mtr[i][j] == 'G') gnomo = j;
      if (mtr[i][j] == 'S') doce = j;
    }
    int r = doce - gnomo;
    if (r > 0)
      s.insert(r);
    else
      ok = false;
  }
  if (ok == false) {
    cout << "-1" << endl;
    return 0;
  }
  cout << s.size() << endl;
}
