Java stringtokenizer. I need a solution

public static void main(String args[]) throws Exception {
    try {
        reader = new FileReader("input.txt");
    } } catch (FileNotFoundException e) {
        System.out.println ("The file cannot be opened."");
        System.exit(1);
    }
    BufferedReader rd = new BufferedReader(reader);
    int cnt_line = 0;// Number of rows in the file
    String line = null;// to save fragments read from the file
    while(((line = rd.readLine())!=null) {// Read one line at a time
        cnt_line++;
    }
    rd.close();

    String sentence[] = new String[cnt_line];// connecting the lines to form a single sentence
    reader = new FileReader("input.txt");
    BufferedReader rd2 = new BufferedReader(reader);
    int a = 0;
    while(((line = rd2.readLine())!=null) {// Read one line at a time
        System.out.println(line);
        sentence[a++] = line;
    }
    rd2.close();

    String op[] = new String[cnt_line];// Contains operators by row
    int n[][] = new int[cnt_line][2];

    StringTokenizer st;
    for (int k = 0; k < sentence.length; k++) {
        System.out.print(sentence[k]);
        st = new StringTokenizer(sentence[k], " ");

        for (int i = 0; i < cnt_line; i++) {
            op[i] = st.nextToken();
            for (int j = 0; j < 2; j++) {
                n[i][j] = Integer.parseInt(st.nextToken());
                if (j == 1)
                    System.out.println(": " + Calc(n[i][0], n[i][1], op[i]));
            }
        }
    }
}

static int Calc(int a, int b, String op) {
    int result = 0;
    switch (op) {
    case "ADD":
        result = a + b;
        break;
    case "SUB":
        result = a - b;
        break;
    case "MUL":
        result = a * b;
        break;
    case "DIV":
        result = a / b;
        break;
    default:
        result = 923;
        break;
    }

    return result;
}