C :

#include<stdio.h>
#include<math.h>
int main()
{
	int i,j;
	int n,a[31],b=0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		scanf("%1d",&a[i]);
		b=b+pow(2,n-1-i)*a[i];
	}
	printf("%d",b);
	return 0;
}

C++ :

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N=2e5+10;
typedef pair<int,int> PII;
typedef long long ll;
typedef priority_queue<int , vector<int>, greater<int>> minqueue;  //从小到大 queue
typedef priority_queue<int, vector<int>, less<int>> maxqueue;      //从大到小 queue

int gcd(int a, int b){  //最大公因数 
    return b ? gcd(b, a % b) : a;
}

int lcm(int a, int b){ //最小公倍数 
    return a * b / __gcd(a, b);
}

int qmi(int base, int power, int p)  //快速幂求余 
{
	int result = 1;   
	while (power > 0)           
	{
		if (power & 1)         							
			result = result * base % p;   
		base = base * base % p ;       						
		power >>= 1;         						
	}
	return result % p;       
}

void solve()
{
	int n;
	string s;
	cin>>n>>s;
	int ans=0;
	for(int i=0;i<s.size();i++){
		if(s[i]=='1') ans+=pow(2,n-1-i);
	}
	cout<<ans<<'\n';
}

signed main()
{
	ios::sync_with_stdio(false); cin.tie(0); cout.tie(0);
	int t;
	t=1;
	//cin>>t; 
	while(t--)     
	{
		solve();
	}
	return 0;
}