#include <iostream>
#include <cstdio>

using namespace std;

int n;
float s;

struct punct
{
    float x,y;
}a[100001];

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

    scanf("%d\n",&n);
    for(int i=0;i<n;i++)
        scanf("%f %f\n",&a[i].x, &a[i].y);
    for(int i=0;i<n-1;i++)
        s=s+(a[i].y+a[i+1].y)*(a[i].x-a[i+1].x)/2;
    s=s+(a[n-1].y+a[0].y)*(a[n-1].x-a[0].x)/2;
    printf("%f\n",s);
    return 0;
}