#include<iostream>
#include<fstream>
#include<math.h>

using namespace std;

int main(){
	ifstream in("aria.in");
	FILE *out=fopen("aria.out","w");
	
	int n; 
	long double xPrev, yPrev, x, y, xF, yF, sol=0;
	in>>n>>xPrev>>yPrev; 
	xF=xPrev; yF=yPrev;
	
	for(int i=1;i<=n-1;i++){
		in>>x>>y;
		sol+=(xPrev*y-x*yPrev);
		xPrev=x; yPrev=y;
	}
	sol+=(xPrev*yF-xF*yPrev);
	sol=sol/2.0;
	//fprintf(out,"%.6Lf",fabs(sol));
	in.close(); fclose(out);
	return 0;
}