I'm bad help me why doesn't this work

[Klukinchikendo]

#include <iostream>
using namespace std;

int main() { 
    int x;
    int a, b;

    cout << "welcome to the calculator" << endl;
    cout << "enter 1 for division, 2 for multiplication, 3 for addition and 4 for subtraction" << endl;
    cin >> x;

    int sum;

    if (x == 1) {
        sum = a / b;
    }

    if (x == 2) {
        sum = a * b;
    }

    if (x == 3) {
        sum = a + b;
    }

    if (x == 4) {
        sum = a - b;
    }

    cout << "enter a number" << endl;
    cin >> a;

    cout << "one more please" << endl;
    cin >> b;

    cout << "and your answer is!" << endl; 
    cout << sum;

    return 0;
}

It just says - 37 or 0. I want it to be able to calculate stuff