#include <bits/stdc++.h>
#define maxN 100002

FILE *fin  = freopen("aria.in", "r" ,stdin);
FILE *fout = freopen("aria.out", "w", stdout);

using namespace std;
int N;
struct Point{ double x, y;} p[maxN];
double sol;

void read(){
    scanf("%d", &N);
    for(int i = 0; i < N; ++ i)
        scanf("%lf %lf", &p[i].x, &p[i].y);
}

int main()
{
    read();
    p[N] = p[0];
    for(int i = 0; i < N; ++ i)
        sol += (p[i].x * p[i + 1].y - p[i + 1].x * p[i].y);
    printf("%.6f\n", sol / 2.0);

    return 0;
}