#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;}