#include <bits/stdc++.h>
using namespace std;
const int MAXN = (int)1010;
set<int> ans;
int n, m;
char a[MAXN][MAXN];
int posk[MAXN];
int posG[MAXN];
int main() {
  cin >> n >> m;
  for (int i = 0; i < n; i++) {
    for (int j = 0; j < m; j++) {
      cin >> a[i][j];
      if (a[i][j] == 'S') {
        posk[i] = j;
      }
      if (a[i][j] == 'G') {
        posG[i] = j;
      }
    }
  }
  for (int i = 0; i < n; i++) {
    if (posG[i] > posk[i]) {
      cout << -1;
      return 0;
    } else {
      ans.insert((int)posG[i] - posk[i] + 1);
    }
  }
  cout << ans.size();
  return 0;
}
