#include<cstdio>
using namespace std;
int n,i,j;
double a,s,v[100001],w[100001];

void det()
{
    s=(v[i]*w[i+1])-(v[i+1]*w[i]);
    a=a+(s/2);
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)scanf("%lf %lf",&v[i],&w[i]);
    v[n+1]=v[1];w[n+1]=w[1];
    for(i=1;i<=n;i++)det();
    printf("%.4lf",a);
}