private static String mkStr(int y, int m, int d){
        String year = Integer.toString(y);
        String month;
        if(m < 10){
            month = "0" + Integer.toString(m);
        }else{
            month = Integer.toString(m);
        }
        String day;
        if(d < 10){
            day = "0" + Integer.toString(d);
        }else{
            day = Integer.toString(d);
        }
        return year + "-" + month + "-" + day;
    }