#include <cstdio>
using namespace std;
int abs(long long x)
    {
    if (x<0)
        return x*(-1);
        else
        return x;
    }
int main()
{
freopen("aria.in","r",stdin);
freopen("aria.out","w",stdout);
long long n,a[100001],b[100001];
scanf("%lld",&n);
long long i,ii,yy;
for (i=1; i<=n; i++)
    scanf("%lld%lld",&a[i],&b[i]);
long long s=0;
for (i=1; i<=n; i++)
    {
    ii=i;
    yy=i+1;
    if (yy>n)
          yy-=n;
    s=s+(a[ii]*b[yy]-a[yy]*b[ii]);
    }
s=abs(s);
printf("%lld",s/2);
if (s%2==1)
    printf(".5");
return 0;
}