#include <cstdio>
#include <map>

int main() {

	long n;

	while (scanf("%ld",&n) == 1) {
		std::map<long,long> ne;
		std::map<long,long> nw;

		long aligned = 0;

		for (long i = 0; i < n; i++) {
			long x,y;
			scanf("%ld %ld",&x, &y);

			long ne_key = y - x;
			if (ne.find(ne_key) != ne.end()) {
				ne[ne_key]++;
			} else {
				ne[ne_key] = 1;
			}

			long nw_key = y + x;
			if (nw.find(nw_key) != nw.end()) {
				nw[nw_key]++;
			} else {
				nw[nw_key] = 1;
			}
		}

		for (std::_Rb_tree_iterator<std::pair<const long, long> > it = ne.begin(); it != ne.end(); ++it) {
			long in_line = it->second;
			aligned += (in_line) * (in_line - 1);
		}

		for (std::_Rb_tree_iterator<std::pair<const long, long> > it = nw.begin(); it != nw.end(); ++it) {
			long in_line = it->second;
			aligned += (in_line) * (in_line - 1);
		}

		long total = n * n;

		printf("%.6lf\n",(double)aligned/(double)total);
	}

	return 0;
}