#include "bits/stdc++.h"
using namespace std;
typedef complex<double> xy_t;
double cross_product(xy_t a, xy_t b) { return (conj(a) * b).imag(); }
#define rep(i, n) for(int i=0;i<(n);i++)
int n;
double x, y;

xy_t P[110];
int main() {
	cin >> n;
	rep(i,n) { 
		cin >> x >> y;
		P[i] = xy_t(x, y);
	}

	double sum = 0.0;
	for (int i = 0; i + 2 < n; i++) {
		xy_t a = P[0], b = P[i + 1], c = P[i + 2];
		sum += cross_product(b - a, c - a) / 2;
	}
	printf("%.1f\n", abs(sum));
}

