#include <cstdio>

using namespace std;

int n,i;
double Sol,x[100002],y[100002];
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) Sol+=x[i]*y[i+1]-x[i+1]*y[i];
    Sol+=x[n]*y[1]-x[1]*y[n];

    if(Sol<0.0) Sol=-Sol;Sol/=2;
    printf("%.5lf",Sol);
    return 0;
}