public static void main ( String [ ] args ) throws IOException {
  FastReader sc = new FastReader ( ) ;
  PrintWriter out = new PrintWriter ( System . out ) ;
  int n = sc . nextInt ( ) , m = sc . nextInt ( ) ;
  int [ ] diff = new int [ n ] ;
  for ( int i = 0 ;
  i < n ;
  i ++ ) {
    String str = sc . next ( ) ;
    int s = str . indexOf ( 'S' ) ;
    int g = str . indexOf ( 'G' ) ;
    if ( s < g ) {
      out . println ( - 1 ) ;
      out . close ( ) ;
      return ;
    }
    diff [ i ] = s - g ;
  }
  Arrays . sort ( diff ) ;
  int count = 0 ;
  while ( true ) {
    int min = 0 ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) {
      if ( diff [ i ] != 0 ) {
        min = diff [ i ] ;
        break ;
      }
    }
    if ( min == 0 ) break ;
    for ( int i = 0 ;
    i < n ;
    i ++ ) diff [ i ] = Math . max ( 0 , diff [ i ] - min ) ;
    count ++ ;
  }
  out . println ( count ) ;
  out . close ( ) ;
}

public static void main ( String [ ] args ) throws IOException { FastReader sc = new FastReader ( ) ; PrintWriter out = new PrintWriter ( System . out ) ; int n = sc . nextInt ( ) , m = sc . nextInt ( ) ; int [ ] diff = new int [ n ] ; for ( int i = 0 ; i < n ; i ++ ) { String str = sc . next ( ) ; int s = str . indexOf ( ' S ' ) ; int g = str . indexOf ( ' G ' ) ; if ( s < g ) { out . println ( - 1 ) ; out . close ( ) ; return ; } diff [ i ] = s - g ; } Arrays . sort ( diff ) ; int count = 0 ; while ( true ) { int min = 0 ; for ( int i = 0 ; i < n ; i ++ ) { if ( diff [ i ] != 0 ) { min = diff [ i ] ; break ; } } if ( min == 0 ) break ; for ( int i = 0 ; i < n ; i ++ ) diff [ i ] = Math . max ( 0 , diff [ i ] - min ) ; count ++ ; } out . println ( count ) ; out . close ( ) ; }
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