#include<cstdio>
#include<cmath>
#define NMAX 100010
using namespace std;

float X[NMAX], Y[NMAX], A;


int main() {
    freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);

	int N;
	scanf("%d", &N);
	
	for (int i = 1 ; i <= N ; i++) 
		scanf("%f %f", &X[i], &Y[i]);

	X[N + 1] = X[1], Y[N + 1] = Y[1];

	for (int i = 1 ; i <= N ; i++) 
		A += X[i] * Y[i+1]  - X[i+1] * Y[i];

	printf("%f\n", fabs(A * 0.5));
    return 0;
}