#include <iostream>
#include <stack>
#include <cmath>



int main(int argc, char const *argv[])
{
	int T;
	std::cin >> T;
	for (int i = 0; i < T; ++i)
	{
		int N;
		std::cin >> N;
		int minimum = 0;
		int min_delta_g = 0x7FFFFFFF;
		int delta_g = 0;
		for(int j = 0; j < N; j++) {
			int G,D;
			std::cin >> G;
			std::cin >> D;
			delta_g += G - D;
			//std::cout << "station " << j << " delta_g: " << delta_g << " minimum:" << min_delta_g << " (id: " << minimum << ")" << std::endl;
			if (delta_g < min_delta_g) {
				minimum = j;
				min_delta_g = delta_g;
			}
		}
		if (delta_g < 0) {
			std::cout << "IMPOSSIBLE" << std::endl;
		} else {
			std::cout << (((minimum + 1) % N) + 1) << std::endl;
		}
	}
	return 0;
}