Why I am getting weird answers?

Question

This code is about evaluation of Postfix expression using stack.

Main class

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String s = scanner.nextLine();
        Solution r = new Solution();
        System.out.println(r.evaluatePostFix(s));
    }
}

Solution class

public class Solution {
    public static int evaluatePostFix(String S) {
        Stack<Character> st = new Stack<>();
        int c1, c2;
        int op = 0;
        for (int i = 0; i < S.length(); i++) {
            char c = S.charAt(i);
            switch(c) {
                case '+':
                    c1 = (int) st.pop();
                    c2 = (int) st.pop();
                    op = c2 + c1;
                    st.push((char) op);
                    break;
                case '-':
                    c1 = (int) st.pop();
                    c2 = (int) st.pop();
                    op = c2 - c1;
                    st.push((char) op);
                    break;
                case '*':
                    c1 = (int) st.pop();
                    c2 = (int) st.pop();
                    op = c2 * c1;
                    st.push((char) op);
                    break;
                case '/':
                    c1 = (int) st.pop();
                    c2 = (int) st.pop();
                    op = c2 / c1;
                    st.push((char) op);
                    break;
                default:
                    st.push(c);
            }

            System.out.println(st);
        }

        return st.peek();
    }
}

What's going wrong? For input 345+*, I got:

[3]
[3, 4]
[3, 4, 5]
[3, i]
[ᓫ]
5355

but 27 is expected!