#include <fstream>

#define  MAX 100000

std::ifstream INPUT_FILE("aria.in");
std::ofstream OUTPUT_FILE("aria.out");

struct punct{
    double x,y;
};

punct puncte[MAX];
int n;
double sum;

void makeSuml(){
    for(int tmp=0;tmp<n;++tmp) sum+=(puncte[tmp].x*puncte[tmp+1].y-puncte[tmp+1].x*puncte[tmp].y);
}

void read(){
    INPUT_FILE>>n;
    for(int tmp=0;tmp<n;++tmp) INPUT_FILE>>puncte[tmp].x>>puncte[tmp].y;
}

int main(){
    read();
    puncte[n]=puncte[0];
    makeSuml();
    OUTPUT_FILE<<sum/2.0;
    return 0;
}