杭电 2036

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#include "stdio.h"typedef struct kks {    int x;    int y;}lists;int main(int argc, char* argv[]){    int index;    lists ms[101];    while (scanf("%d",&index))    {        if (index == 0 )        {            break;        }        else        {            for (int i = 0 ; i < index ; ++i )            {                scanf("%d%d",&ms[i].x ,&ms[i].y);            }            double m = 0;            m = ms[index-1].x * ms[0].y - ms[index-1].y * ms[0].x ;            for ( i = 0 ; i < index - 1 ; ++i )            {                m += ms[i].x * ms[i+1].y - ms[i].y * ms[i+1].x;            }            m = m / 2 ;            printf("%.1f\n",m);        }    }    return 0;}