#include <iostream>
#include <cstdio>
#include <vector>

using namespace std;

#define Nmax 100005

struct punct{

    double x;
    double y;

} V[Nmax];

int N;
double S;

void citire(){

    freopen("aria.in", "r", stdin);

    scanf("%d", &N);

    for ( int i = 1; i <= N; ++i )
        scanf("%lf %lf", &V[i].x, &V[i].y);

    fclose(stdin);
}

void rezolva(){

    V[N + 1] = V[1];

    for ( int i = 1; i <= N; ++i )
        S += ( V[i].x * V[i + 1].y - V[i + 1].x * V[i].y );

    S /= 2.0;
}

void afis(){

    freopen("aria.out", "w", stdout);

    printf("%.10lf\n", S);

    fclose(stdout);
}

int main(){

    citire();
    rezolva();
    afis();

    return 0;
}