Gray to binary code

/** Gray code to binary code
Minimum number of operations required to convert a binary number to 0 using the method:
change ith binary digit only if (i+1)th binary digit is 1 and all digits following that is 0.
**/

#include <bits/stdc++.h>
#define ll long long
using namespace std;


int main() {
    string S;
    cin>>S;
    ll fact = 1;
    int prev = S[0]-'0';
    ll ans = prev;
    for(ll i=1; i<S.size(); ++i){
        prev = prev^(S[i]-'0');
        ans = ans*2 + prev;
    }
    cout<<ans;
    return 0;
}