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#include <stdio.h>
#include <math.h>
#define pi 3.1415926
int main() {
double x,a,b,m,answer;
scanf("%lf",&x);
m=0.000001;
a=0;
b=pi;
answer=pi/2;
while(fabs(cos(answer)-x)>m)
{
if(cos(answer)-x>m){
a=answer;//b=answer;
}
else{
b=answer;//a=answer;
}
answer=(a+b)/2;
}
printf("%.5lf",answer);
return 0;
}