What the best way to buffer overflow this and run multiplication instead of addition

Hey, I’m new to buffer overflow exploits(just started 3 days ago). I’m trying to run this code and instead of the addition function running “+” run the multiplication function “*” using buffer overflow. I have tried multiple different but non prevailed

#include <stdio.h>


int num1;
int num2;
int res;

static int add(void)
{
    res = num1+num2;
    printf("\nAnswer: %u", res);
}
static int sub(void)
{
    res = num1-num2;
    printf("\nAnswer: %u", res);
}
static int mal(void)
{
    res = num1*num2;
    printf("\nAnswer: %u", res);
}
static int div(void)
{
    res= num1/num2;
    printf("\nAnswer: %u", res);
}



void do_operator(){

    char op[1];

    printf("Enter an operator (+, -, *, /):");
    scanf("%s", &op);

    if(*op == '+')
    {
        add();
    }
    else if (*op == '*')
    {
        mal();
    }
        else if (*op == '-')
    {
        sub();
    }
    else if (*op == '/')
    {
        div();
    }
    else
    {
        printf("\nNo valid input: %s", op);
    }

}


int main()
{
    printf("\nEnter number-1:");
    scanf("%u", &num1);
    
    printf("\nEnter number-2:");
    scanf("%u", &num2);
    
    do_operator();
    return 0;
}