POJ 1654 Area 多边形面积 G++会WA

#include<stdio.h>
#include<algorithm>
#include <cstring>
using namespace std;

typedef long long ll;
const int MAXN = 1000008;
char s[MAXN];
int dx[] = {-1, -1, -1, 0, 0, 0, 1, 1, 1};
int dy[] = {-1, 0, 1, -1, 0, 1, -1, 0, 1};

int main()
{
//    freopen("in.txt","r",stdin);
    int T;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%s", s);
        int len = strlen(s);
        int x, y, nx, ny;
        x = y = 0;
        ll ans = 0;
        for(int i=0; i<len; i++)
        {
            nx = x + dx[s[i]-'1'];
            ny = y + dy[s[i]-'1'];
            ans += x*ny - y*nx;
            x = nx;
            y = ny;
        }
        if(ans < 0) ans = -ans;
        printf("%I64d", ans / 2);
        if(ans % 2 == 1) printf(".5");
        puts("");
    }
    return 0;
}