Modify a stack such a way that it should return smallest element in O(1) complexity

import java.util.Stack;

public class MinimumFromStack {
    public static void main(String[] args) {
        Stack<Pair> stack = new Stack<>();
        Pair pair = new Pair(5,stack);
        stack.add(pair);
        Pair pair1 = new Pair(9,stack);
        stack.add(pair1);
        Pair pair2 = new Pair(7,stack);
        stack.add(pair2);
        Pair pair3 = new Pair(6,stack);
        stack.add(pair3);
        Pair pair4 = new Pair(3,stack);
        stack.add(pair4);
        
        
        System.out.println("Smallest element in stack : " + stack.peek().getMinimum());
        System.out.println("Top element in stack : " + stack.peek().getActual());
        System.out.println("removing Top element in stack : " + stack.pop());
        System.out.println("Smallest element in stack : " + stack.peek().getMinimum());
    }
    
    private static class Pair {
        int actual;
        int minimum;
        
        Pair(int actual,Stack<Pair> stack){
            this.actual =actual;
            this.minimum = stack.isEmpty() ? actual : Math.min(actual,stack.peek().getMinimum());
        }
        
        public int getActual() {
            return actual;
        }
        
        public int getMinimum() {
            return minimum;
        }

    }
}

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