#include <iostream>
#include <cstdio>

using namespace std;
double x[100001],y[100001];
int main()
{freopen("aria.in","r",stdin);
 freopen("aria.out","w",stdout);
double s;
int n,i;
scanf("%d",&n);
for(i=1;i<=n;i++)
    {scanf("%lf",&x[i]);
     scanf("%lf",&y[i]);
    }
x[n+1]=x[1];
y[n+1]=y[1];
for(i=1,s=0;i<=n;i++)
    s+=(x[i]*y[i+1]-x[i+1]*y[i]);
if(s<0)
    printf("%lf",-s/2);
else
    printf("%lf",s/2);
    return 0;
}