#include <bits/stdc++.h>

using namespace std;

double x[100007],y[100007],s;

int main()
{
    int i,n;
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d", &n);
    for (i=1;i<=n;i++)
        scanf("%lf%lf", &x[i], &y[i]);
    x[++n]=x[1];
    y[n]=y[1];
    for (i=2;i<=n;i++)
        s+=(x[i-1]*y[i]-x[i]*y[i-1]);
    printf("%lf", s/2.0);
    return 0;
}