Why WA in lastdig2?

[adithecool]

#include<stdio.h>
#include<math.h>
main()
{
	int n,i,v,b,a;
   long long int x;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		scanf("%d%d",&a,&b);
		if(a==0&&b==0)
		x=0;
		else{
		v=b%4;
		if(v==0&&b!=0)
		v=4;
		a=a%10;
		x=pow(a,v);
		x=x%10;}
		printf("%d\n",x);
	}
	}

[leppyr64]

Read the input constraints