#include <bits/stdc++.h>   using namespace std;   using ll = long long;   int main() {     ios::sync_with_stdio(0), cin.tie(0);     ;     int n, m;     cin >> n >> m;     vector<vector<char>> v(n, vector<char>(m));     for (int i = 0; i < n; i++)       for (int j = 0; j < m; j++) cin >> v[i][j];     bool st = 1;     int mx = 0;     set<int> se;     for (int i = 0; i < n; i++) {       int s = -1, g = -1;       for (int j = 0; j < m; j++) {         if (v[i][j] ==  G )           g = j;         else if (v[i][j] ==  S )           s = j;       }       if (s < g)         st = 0;       else         se.insert(s - g);     }     if (st)       cout << se.size();     else       cout << -1;   }   