C++ Program:

Example:

#include <iostream>
#include <math.h>
using namespace std;

int toDecimal(long long num) {
  int n=0,i=0;
  while(num!=0) {
    if(num%10==0 || num%10==1) {
      n=n+pow(2,i++)*(num%10);
    }
    else {
      cout<<"Number is not in binary form \n";
      return 0;
    }
    num/=10;
  }
  return n;
}

int main() {
  long long num;
  int count=0;
  cout<<"Enter Your binary number : ";
  cin>>num;
  cout<<"Number in decimal : "<<toDecimal(num);
  return 0;
}

Output:

  Enter Your binary number : 1101
  Number in decimal : 13

Binary Numbers : Numbers with only 0 & 1 combination

Examples like: 101, 111, 10, 11111

Main Logic :

int toDecimal(long long num) {
  int n=0,i=0;
  while(num!=0) {
    if(num%10==0 || num%10==1) {  // checking for binary
      n=n+pow(2,i++)*(num%10);   // converting  to decimal
    }
    else {
      cout<<"Number is not in binary form \n";
      return 0;
    }
    num/=10;
  }
  return n;
}

  Calculation:
  Lets take binary number n=100

  = 20*0 + 21*0 + 22*1
  = 0 + 0 + 4
  = 4

------------------------------------------------------------------------
  take every single digit of your binary number from right side and
  multiply it with 2 to the power of index position 
  starting form 0 at the right side 

  Index position  2 1 0
                  | | |
  Binary Number   1 0 0