#include<stdio.h>
#include<cs50.h>

int main(void)
{
		long long ccnum;
		do{
			printf("Enter a number: ");
		    ccnum = get_long_long();
		}
		while(ccnum<0);

		//"count" variable to count no. of digits in ccnumber
		int count=0;
		long num=ccnum;

		//this while loop counts no.of digits in ccnumber
		while(num>0)
		{
			num=num/10;
			count++;
		}

		//below if condn checks  ccnumber contains how many digits and sees if valid or not
		if(count!=15 && count!=13 && count!=16)
		{
			printf("INVALID\n");
		}
        else{
		//below array stores the ccnum in reverse order =>1 digit each
		int number[count];
		for(int i=0;i<count;i++)
		{
			number[i]=(int) (ccnum%10);
			ccnum=ccnum/10;
		}

		//below array stores the same reverse array by eliminating 1st num and makes it '0' kind of empty
		int orignumber[count];
		for(int i=1;i<count;i++)
		{
			orignumber[i]=number[i];
		}

		//below process multiplies '2' to the 'number' array elements in index 1,3,5...so on and
		//if any of it is double digit it makes it single als0(while loop)
		int digit =0;
		for(int i=1;i<count;i+=2)
		{
			number[i]*=2;
			while(number[i]>9)
			{
				digit=number[i]%10;
				number[i]=number[i]/10;
				number[i]=number[i]+digit;
			}

		}
		//this process adds all the digits which are multiplied by 2 and total is called 'sum2nd'
		int sum2nd=0;
		for(int i=1;i<count;i+=2)
		{
			sum2nd=sum2nd+number[i];
		}

		//this process adds 'sum2nd' and the remaining elements which 'not' multiplied by 2
		int checksum=sum2nd;
		for(int i=0;i<count;i+=2)
		{
			checksum=checksum+number[i];
		}
		//this checks if the totalsum(checksum) ends in zero or not
		if(checksum%10==0)
		{

		//visa starts with 4
		if(orignumber[count-1]==4)
		{
			printf("VISA\n");
		}
		//visa starts with 4
		else if((orignumber[count-2]==7) && (orignumber[count-1]==4||orignumber[count-1]==3))
		{
			printf("AMEX\n");
		}
		//Amex starts with 34,37
		else if((orignumber[count-1]==5) && (orignumber[count-2]==1||orignumber[count-2]==2||orignumber[count-2]==3||orignumber[count-2]==4||orignumber[count-2]==5))
		{
			printf("MASTERCARD\n");
		}
		//master card starts with 51,52,53,54,55
		else
		{
			printf("INVALID\n");
		}
		}
		else
		{
			printf("INVALID\n");
		}
        }
	}