#include <bits/stdc++.h>
using namespace std;
struct my {double x,y;} V[100100];
double sol;
int N;
int main(){
	ios_base::sync_with_stdio(0);
	cin.tie(0);
	cin >> N;
	for(int i = 0;i<N;i++) cin >> V[i].x >> V[i].y;
	V[N] = V[0];
	for(int i = 1;i<=N;i++) sol+=V[i-1].x*V[i].y - V[i-1].y*V[i].x;
	cout.precision(10);
	cout << fixed << abs(sol/2.0);
	return 0;
}