#include <cstdio>
#include <vector>
using namespace std;
#define MAXN 100005

struct pc
{
    int x,y;
}crt;

double aria(pc A, pc B, pc C)
{
    double r = A.x*B.y + A.y*C.x + B.x*C.y - C.x*B.y -A.x*C.y -A.y*B.x;
    r= r / (double) 2;
    return r;
}

vector<pc> v;
int n,i,x,y;
double sum;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d", &n);

    for(i=1; i<=n; i++)
    {
        scanf("%d%d",&crt.x,&crt.y);
        v.push_back(crt);
    }

    for(i=1; i<v.size()-1; i++)
        sum += aria(v[0], v[i], v[i+1]);

    printf("%.5lf", sum);
}