$2.($P$132$: $6)$ Write a program that reads a $4-$bit binary number from the keyboard as a string and then converts it into decimal. For example, if the input is $1100,$ the output should be $12.($Hint: Break the string into substrings and then convert each substring to a value for a single bit. If the bits are b$0,$ b$1,$ b$2,$ and b$3,$ the decimal equivalent is $8$b$0+4$b$1+2$b$2+$b$3).$