#include <cstdio>
#include <cmath>

#define NMAX 100007

using namespace std;

int n, X[NMAX], Y[NMAX];
double Sum;

int main(){
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i)
        scanf("%d %d", &X[i], &Y[i]);
    X[n + 1] = X[1];
    Y[n + 1] = Y[1];
    for(int i = 1; i <= n; ++i)
        Sum += (X[i] * Y[i + 1] - X[i + 1] * Y[i]);
    printf("%lf", fabs(Sum / 2.0));
    return 0;
}