Why the answer is always -1

Question

#include <iostream>
using namespace std;
int digit(long,int);
int main()
{
long n;
int k;
cin>>n>>k;
cout<<digit(n,k)<<endl;
return 0;

}
int digit(long n , int k)
{
int a,b,c,e;
for (b=0;a>0;b++)
{a=n/10;}
int NUM[b+1];
for (c=1;c<=b;c++)
{NUM[c]=n%10;}
if (k<=b)
e=NUM[k];
else e=-1;
return e;

}