#include <iostream>

#include <vector>

#include <string>

#include <algorithm>

#include <set>

using namespace std;

int main(){

    int n,m;

    cin >> n >> m;

    set<int> distance;

    bool flag = true;

    for(int  i =  ; i < n; ++ i){

        string rectangle;

        cin >>  rectangle;

        int dwarf_pos = rectangle.find('G');

        int candy_pos = rectangle.find('S');

        int dist = candy_pos - dwarf_pos;

        if(dist > ) distance.insert(dist);

        else flag = false;

    }

    if(!flag) cout<<-<<endl;

    else cout<<distance.size()<<endl;

    return ;

}