#include<stdio.h>
#include<math.h>
float f(float x)
{float y;
y=x*x+2*x+1;
return y;
}
main()
{float a,b,c,x;
printf("%s","input three numbers:\n");
scanf("%f%f%f",&a,&b,&c);
while(fabs(b-a)>=c)
{x=(a+b)/2;
if(f(x)!=0)
{if(f(a)*f(x)<0)
b=x;
else a=x;}
else a=x,b=x;
}
printf("%f\n",(a+b)/2);
}
