Rational Number Operations using Pointers

#include <stdio.h>
#include <stdlib.h>
struct rational{
        int numerator;
        int denominator;
}*n3 = NULL; //n3 is the final answer
struct rational *insert(struct rational *n, int a, int b){
        n = (struct rational*) malloc(sizeof(struct rational)); //memory is allocated for inserting values
        n->numerator = a;
        n->denominator = b;
        return n;
}
void add(struct rational *n1, struct rational *n2){ //Formula for addition is given
        int a = (n1->numerator)*(n2->denominator) + (n1->denominator)*(n2->numerator);
        int b = n1->denominator * n2->denominator;
        n3 = insert(n3, a, b); //resulting number will have a numerator and denominator
}
void multiply(struct rational *n1, struct rational *n2){ //formula for multiplication is given
        int a = n1->numerator * n2->numerator;
        int b = n1->denominator * n2->denominator;
        n3 = insert(n3, a, b);  //resulting number will have a numerator and denominator
}
int main(){
        int a, b, choice;
        struct rational *n1 = NULL, *n2 = NULL; //good practict to initialize pointers to NULL
        start: printf("Enter the numerator of first number: ");
        scanf("%d", &a);
        printf("Enter the denominator of first number: ");
        scanf("%d", &b);
        n1 = insert(n1, a, b); //n1 will contain a numerator and a denominator
        printf("Enter the numerator of second number: ");
        scanf("%d", &a);
        printf("Enter the denominator of second number: ");
        scanf("%d", &b);
        n2 = insert(n2, a, b); //n2 will also contain numerator and denominator
        while(1){
                printf("\nThe first number is %d/%d", n1->numerator, n1->denominator);
                printf("\nThe second number is %d/%d", n2->numerator, n2->denominator);
                printf("\n1.Addition 2.Multiplication 3.Different numbers 4.Exit: ");
                scanf("%d",&choice);
                switch(choice){
                        case 1: add(n1, n2);
                                    printf("Addition is %d/%d", n3->numerator, n3->denominator);
                                    break;
                        case 2: multiply(n1, n2);
                                    printf("Multiplication is %d/%d", n3->numerator, n3->denominator);
                                    break;
                        case 3: goto start;
                        case 4: return 0;
                }
        }
}