#include <stdio.h>#include <math.h>#define MAXN 1000typedef struct nDate{double x, y;}nNode;nNode nP[MAXN];int nNum;double ans;double funcSolve();double procGetArea(int i, int j, int k);int main(){int i;while (1 == scanf("%d", &nNum), nNum){for (i = 0; i != nNum; ++i){scanf("%lf %lf", &nP[i].x, &nP[i].y);}printf("%.lfn", funcSolve() / 2);}return 0;}double funcSolve(){int i;double nArea = 0;if (nNum < 3) return 0;nNum -= 2;for (i = 0; i != nNum; ++i){nArea += procGetArea(0, i + 1, i + 2);}if (nArea < 0) nArea = - nArea;return nArea;}double procGetArea(int i, int j, int k){double s = (nP[k].y - nP[i].y) * (nP[j].x - nP[i].x) - (nP[j].y - nP[i].y) * (nP[k].x - nP[i].x); return s; }
