26 lines
544 B
C
26 lines
544 B
C
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#include <stdio.h>
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#define ABS(x) ((x>0)?(x):(-x))
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struct point{
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long x;
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long y;
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};
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int main(void){
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int numbers = 0;
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struct point points[128];
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while(scanf("%d",&numbers) == 1 && numbers){
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int i;
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double s = 0.0;
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for(i = 0; i < numbers; i++){
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scanf("%ld",&points[i].x);
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scanf("%ld",&points[i].y);
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}
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for(i = 0; i < numbers-1; i++)
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s += points[i].x * points[i+1].y - points[i].y * points[i+1].x;
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s += points[numbers-1].x * points[0].y - points[numbers-1].y * points[0].x;
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printf("%.1lf\n",0.5*ABS(s));
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}
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return 0;
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}
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