#include<stdio.h>
int i,n;
double x[100001],y[100001],s;
int main() {
    freopen("aria.in","r",stdin),freopen("aria.out","w",stdout),scanf("%d",&n);
    for(i=0;i<n;++i)
        scanf("%lf%lf",&x[i],&y[i]);
    for(i=0;i<n-1;i++)
        s+=(x[i]*y[i+1]-x[i+1]*y[i])/2;
    printf("%.5lf",s+((long double)x[n-1]*y[0]-(long double)x[0]*y[n-1])/2);
}