#include<cstdio>
using namespace std;
int n,i;
double x[100007],y[100007],s;
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]);
    for(i=1;i<n;i++)
        s+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    s+=((long double)x[n]*y[1]-(long double)x[1]*y[n])/2;
    printf("%.5lf",s);
}