小X的多边形(牛客\必会求解凸形面积知识)

小X的多边形

#include <bits/stdc++.h>
using namespace std;

signed main(){
    int n; cin >> n;
    int a[10][2];
    for(int i = 0; i < n; ++i){
        for(int j = 0; j < 2; ++j){
            cin >> a[i][j];
        }
    }
    a[n][0] = a[0][0];
    a[n][1] = a[0][1];
    int sum = 0;
    for(int i = 0; i < n; ++i){
        sum += a[i][0] * a[i+1][1] - a[i][1]*a[i+1][0];
    }
    printf("%d\n", (sum + 1) / 2);
    
    return 0;
}