#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

// mmap
#include <sys/mman.h>
// write
#include <unistd.h>
// exit
#include <stdio.h>
#include <stdlib.h>

// clang-format off
#define RD(v) int v=0;{int _c;while(_c=*rp++-48,_c>=0)v=v*10+_c;} //
#define RD2(v) {int _c;int sign=1;if(*rp=='-'){sign=-1;rp++;}while(_c=*rp++-48,_c>=0)v=v*10+_c;v*=sign;} //
// clang-format on

int x[30];
int y[30];

int main() {
    // void *mmap(void * addr , size_t length , int prot , int flags , int fd , off_t offset );
    char *rp = (char *)mmap(0l, 1l << 28, 1, 2, 0, 0ll);
    RD(n);
    for (int i = 0; i < n; ++i) {
        RD2(x[i]);
        RD2(y[i]);
        printf("%d %d\n", x[i], y[i]);
    }
    int ans = 0;
    for (int i = 0; i < n; ++i) {
        ans += (x[i] - x[(i + 1) % n]) * (y[i] + y[(i + 1) % n]);
    }
    if (ans < 0) ans = -ans;
    printf("%d\n", ans);
    exit(0);
    return 0;
}