#include <iostream>
#include <string>
 
using namespace std;
 
unsigned int pow(unsigned int a, unsigned int n);
 
int main()
{
    string binary;
    unsigned int decimal = 0;
 
    cout << "Enter a binary number: ";
    cin >> binary;
 
    for(int i = binary.length()-1; i >= 0; i--) {
        if(binary[i] > '1' || binary[i] < '0') {
            cout << "Invalid binary number!" << endl;
            return 1;
        } else {
            decimal += (binary[i] - '0') * pow(2, (binary.length()-i-1));
        }
    }
 
    cout << "Decimal: " << decimal << endl;	
    return 0;
}
 
unsigned int pow(unsigned int a, unsigned int n)
{
    unsigned int r = 1;
    while( n-- > 0 ) r *= a;
    return r;
}