making my CS50 solution more elegant and efficient - c

Question

I am taking CS50, and I completed the first week problem of implementing the Luhn Check algorithm. Although my solution works, I feel it's very sloppy and not efficient. Can someone please suggest how I can improve my solution. (It's a first week problem; you are not intended to use arrays or functions.)
Here is the code:
#include<stdio.h>
#include<stdlib.h>
#include<cs50.h>
int main()
{
long long x,j=0;
int len,i,len_flag,flag=0,flag_val=0,sp_len=0,check_flag=0; //basic integers related to loop etc
int res,sum,fin_sum=0,h=0,d2_sum=0,d1_sum=0,ff_num=0; // variables related to extracting numbers
int sum_len=0,in_sum=0,in_fsum=0,len_sum=0,m=0; // extraing numbers from more than single sum result
int sum_f=0,sum_final=0;
do{
x = get_long("enter a valid card number \n");
j=x;
len_flag = 0;
len = 0;
while(len_flag!=1)
{
x = x / 10;
if(x==0)
{
len_flag = 1; //finding the lenght of the number
}
len++;
}
if(len==15||len==16||len==13)
{
flag = 1;
sp_len =1;
}
else
{
flag=1;
}
}while(flag!=1);
for(i=0;i<len;i++)
{
res = j % 10;
j = j / 10;
if(i%2!=0)
{
sum = res * 2;
//printf("multi_res : %d \n",sum);
len_flag = 0;
sum_len = 0;
len_sum = sum;
while(len_flag!=1)
{
len_sum = len_sum / 10;
if(len_sum==0)
{
len_flag=1; // checking if the sum is more than single digit
}
sum_len++;
//printf("trig\n");
}
if(sum_len>1)
{ x=0;
while(x<sum_len)
{
in_sum = sum % 10;
sum = sum/10;
in_fsum = in_fsum + in_sum;
//printf("double sum : %d \n",in_fsum);
x++;
}
}
fin_sum = fin_sum + sum;
}
if(i==len-1)
{
for(h=0;h < 1;h++)
{
fin_sum = fin_sum + in_fsum;
}
d1_sum = res;
}
else if(i%2==0)
{
sum_f = sum_f + res; // adding up the number that where not x2
}
if(i==len-2)
{
d2_sum = res; //5555555555554444
}
}
sum_final = sum_f + fin_sum;
//printf("sum_final : %d\n",sum_final);
//printf("sum_f_final : %d\n",sum_f);
//printf("sum_in_final : %d\n",fin_sum);
if(sum_final%10==0)
{
flag_val=1; // checking if the number is valid
}
if(ff_num == 0)
{
ff_num = (d1_sum*10) + d2_sum; //flip
}
do
{
if(sp_len==0)
{
printf("INVALID\n");
check_flag=1;
break;
}
if((flag==1&&flag_val==1&&ff_num==34)||ff_num==37)
{
printf("AMEX\n");
check_flag=1;
break;
}
else if(flag==1&&flag_val==1&&ff_num>50&&ff_num<56)
{
printf("MASTERCARD\n");
check_flag=1;
break;
}
else if(flag==1&&flag_val==1&&d1_sum==4)
{
printf("VISA\n");
check_flag=1;
break;
}
else
{
printf("INVALID\n");
check_flag=1;
break;
}
}while(check_flag!=1);
return 0;
}
I felt like I was fixing a leak while writing the code. I would try to correct one thing, and another thing would go wrong, and this is the final result.