#include <iostream>
#include <stdio.h>
using namespace std;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    int n, x1, x2, y1, y2, xi, yi;
    float ap=0;
    cin>>n;

    scanf("%d %d", &xi, &yi);
    x1=xi;
    y1=yi;

    for(int i=2; i<=n; i++)
    {
        scanf("%d %d \n", &x2, &y2);
        ap+=((x1*y2)-(x2*y1))/2;
        x1=x2;
        y1=y2;
    }

    ap+=((x2*yi)-(xi*y2))/2;

    cout<<ap;




    return 0;
}