Variable declaration inside curly braces

Why does the following code produce an error? I don't understand why the curly braces are making a difference.

#include<stdio.h>

int main(void)
{
    {
        int a=3;
    }

    {
        printf("%d", a); 
    }

    return 0;
}

The scope of a local variable is limited to the block between {}.

In other words: outside the block containing int a=3; a is not visible.

#include<stdio.h>
int main()
{
    {
      int a=3;
      // a is visible here
      printf("1: %d", a);  
    }

    // here a is not visible
    printf("2: %d", a);  

    {
     // here a is not visible either
      printf("3: %d", a); 
    }

    return 0;
}

Hint: google c scope variables