Multiplication of large numbers in c

#include<stdio.h>

#include<math.h>

#include<stdlib.h>

#include<string.h>

#define MAX 10000

char * multiply(char [],char[]);

int main(){

    char a[MAX];

    char b[MAX];

    char *c;

    int la,lb;

    int i;

    printf("Enter the first number : ");

    scanf("%s",a);

    printf("Enter the second number : ");

    scanf("%s",b);

    printf("Multiplication of two numbers : ");

    c = multiply(a,b);

    printf("%s",c);

    return 0;

}

char * multiply(char a[],char b[]){

    static char mul[MAX];

    char c[MAX];

    char temp[MAX];

    int la,lb;

    int i,j,k=0,x=0,y;

    long int r=0;

    long sum = 0;

    la=strlen(a)-1;

        lb=strlen(b)-1;

   

        for(i=0;i<=la;i++){

                a[i] = a[i] - 48;

        }

        for(i=0;i<=lb;i++){

                b[i] = b[i] - 48;

        }

    for(i=lb;i>=0;i--){

         r=0;

         for(j=la;j>=0;j--){

             temp[k++] = (b[i]*a[j] + r)%10;

             r = (b[i]*a[j]+r)/10;

         }

         temp[k++] = r;

         x++;

         for(y = 0;y<x;y++){

             temp[k++] = 0;

         }

    }

   

    k=0;

    r=0;

    for(i=0;i<la+lb+2;i++){

         sum =0;

         y=0;

         for(j=1;j<=lb+1;j++){

             if(i <= la+j){

                 sum = sum + temp[y+i];

             }

             y += j + la + 1;

         }

         c[k++] = (sum+r) %10;

         r = (sum+r)/10;

    }

    c[k] = r;

    j=0;

    for(i=k-1;i>=0;i--){

         mul[j++]=c[i] + 48;

    }

    mul[j]='\0';

    return mul;

}