#include<stdio.h>
int i,n;
double x[100000],y[100000],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(x[n]=x[0],y[n]=y[0],i=0;i<n;i++)
        s+=((long double)x[i]*y[i+1]-(long double)x[i+1]*y[i])/2;
    printf("%.5lf",s<0?-s:s);
}