#include<stdio.h>

int n;
double x[100001],y[100001];
double A;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf %lf",&x[i],&y[i]);
    x[0] = x[n];
    y[0] = y[n];

    for(int i=0;i<n;i++)
        A+=( x[i]*y[i+1] - x[i+1]*y[i]  );
    if(!A)
        A = -A;
    A/=2;
    printf("%lf",A);

}