#include <cstdio>
#include <iostream>
#define M 100010
using namespace std;

double x[M],y[M],aria;
int i,n;
int main()
{
    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+1]=x[1];y[n+1]=y[1];
    for(i=1;i<=n;i++)
        aria+= (x[i]-x[i+1])*(y[i]+y[i+1]);//x[i]*y[i+1]-y[i]*x[i+1];
    if(aria<0.0)aria = -aria;
    aria/=2.0;
    printf("%.5lf",aria);
    return 0;
}