#include <bits/stdc++.h>

int ri() {
	int n;
	scanf("%d", &n);
	return n;
}

int main() {
	int n = ri();
	std::pair<int, int> a[n];
	for (auto &i : a) i.first = ri(), i.second = ri();
	int64_t res = 0;
	for (int i = 1; i + 1 < n; i++) {
		res += std::abs((a[0].first - a[i].first) * (a[i].second - a[i + 1].second) - (a[0].second - a[i].second) * (a[i].first - a[i + 1].first));
	}
	printf("%" PRId64 "\n", res);
	return 0;
}