CF 9C水题

输入一个n，求小于n只有1和0组成的数的个数

#include<cstdio>
#include<cmath>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
#define INF 0x3f3f3f3f
typedef long long ll;
const double eps=1e-8;
int sgn(double a){return a<-eps?-1:a<eps?0:1;}
int main()
{
    char s[11];scanf("%s",s);
    int len=strlen(s);
    int ans=0;
    for(int i=0;i<len;i++)
    {
        if(s[i]>'1')
        {
            ans+=(1<<(len-i))-1;
            break;
        }
        if(s[i]=='1')
            ans+=(1<<(len-i-1));
    }
    printf("%d",ans);
}