#include <fstream>
#include <cstdio>
using namespace std;
ifstream f("aria.in");
FILE*g = fopen("aria.out","w");
int n, i, j, k;
double x[100010], y[100010], a;
int main(){
    f>>n;
    for(i=1; i<=n; i++)
		f>>x[i]>>y[i];
	f.close();
	for(i=1; i<n; i++)
		a=a+(x[i]*y[i+1]-x[i+1]*y[i]);
	a=a+(x[n]*y[1]-x[1]*y[n]);
	if(a<0)
		a=-a;
	fprintf(g,"%.5lf",a/2.0);

	fclose(g);
    return 0;
}