#include<fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");

double n,aria,x1,y1,xx,yy,x2,y2;

int main(){
    in>>n;
    in>>xx>>yy;
    x1=xx;y1=yy;
    for(int i=1;i<n;++i){
        in>>x2>>y2;
        aria+=x1*y2-x2*y1;
        x1=x2;y1=y2;
    }
    aria+=yy*x2-xx*y2;
    out<<aria/2;
    return 0;
}