cogs 1229. 多边形面积

叉乘的性质

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;

int n,x[406],y[406];
int ans;

int main(){
    freopen("areas.in","r",stdin);
    freopen("areas.out","w",stdout);
    scanf("%d",&n);
    for(int i=1;i<=n;++i)
        scanf("%d%d",&x[i],&y[i]);
    for(int i=2;i<=n;++i)
        ans+=(x[i-1]*y[i]-x[i]*y[i-1]);
    ans+=(x[n]*y[1]-x[1]*y[n]);
    printf("%d",ans/2);
    //while(1);
    return 0;
}