#include <stdio.h>
#define N 100000
#define fr(i,a,b) for(int i=a;i<b;++i)
double x[N],y[N];
int main(){
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n;
    scanf("%i\n",&n);
    fr(i,0,n) scanf("%lf%lf",x+i,y+i);
    double area=0;
    fr(i,0,n){
        int j=i+1;if(j==n)j=0;
        area+=x[i]*y[j]-x[j]*y[i];
        }
    if(area<0) area=-area;
    area/=2;
    printf("%lf",area);
    return 0;
    }