#include <cstdio>
#define nMax 101024

using namespace std;

struct Point{
    double x,y;
};

int n;
double surface;
Point points[nMax];

double abs(double x) {
    if(x<0) {
        return -x;
    }
    else {
        return x;
    }
}

void scanPt(Point&P) {
    scanf("%lf%lf",&P.x,&P.y);
}

void read() {
    int i;
    scanf("%d",&n);
    for(i=0;i<n;++i) {
        scanPt(points[i]);
    }
}

int surf(Point P1,Point P2) {
    return P1.x*P2.y-P2.x*P1.y;
}

void solve() {
    int i;
    surface+=surf(points[n-1],points[0]);
    for(i=1;i<n;++i) {
        surface+=surf(points[i-1],points[i]);
    }
}

void display() {
    printf("%.7f",abs(surface)/2);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    read();
    solve();
    display();
    return 0;
}