#include <stdio.h>

const char* ts(unsigned long long n)
{
	static char b[27];
	sprintf(b, "%llu", n);

	char *p = b;
	while ( *p )
		++p;

	char* q = p + (p - b - 1) / 3;
	for ( short c = 0; p < q; ++c )
	{
		*q-- = *p--;
		if ( c && c % 3 == 0 )
			*q-- = ',';
	}

	return b;
}

int main()
{
	printf("[%s]\n", ts(1234567890ull));
	return 0;
}