#include <stdio.h>
#include <math.h>

double shori(double a, double b, double eps )
{
   double c ;
    while( b-a > eps ){
       c = ( a + b )/2.0 ;
       if( cos(a) * cos(c) <0 ) {
           b = c;
       }else{
           a = c;
       }
   }
   c=( a + b )/2.0 ;
   return(c);
}

main()
{
   double eps ;
   double a, b ;
   double c ;

   printf("input a \n");
   scanf("%lf", &a );

   printf("input b \n");
   scanf("%lf", &b );

   if( b<=a || cos(a)*cos(b)>0.0 ){
      fprintf( stderr, "illegal interval %g %g\n", a, b );
      exit(1);
   }

   printf("input epsilon \n");
   scanf("%lf", &eps );

   if( eps<=0 ){
      fprintf( stderr, "illegal accuracy %g\n", eps );
      exit(1);
   }
   c = shori( a, b, eps );
   printf( "%g (f(c)=%g)\n", c , cos( c ) );
}