大佬们，看一下这道题哪错了，为啥只有50分呢？

P1055 [NOIP2008 普及组] ISBN 号码

#include <iostream>
#include <string.h>
using namespace std;
int main()
{
    char str[20];
    cin >> str;
    int len = strlen(str);
    int sum = 0;
    double remainder = 0;
    sum = (str[0] - '0') * 1 + (str[2] - '0') * 2 + (str[3] - '0') * 3 + (str[4] - '0') * 4 + (str[6] - '0') * 5 + (str[7] - '0') * 6 + (str[8] - '0') * 7 + (str[9] - '0') * 8 + (str[10] - '0') * 9;
    remainder = sum % 11;
    int a = str[12]-'0';
    if (remainder == a)
    {
        cout << "Right";
    }
    else
    {
        for (int i = 0; i <= 11; i++)
        {
            cout << str[i];
        }
        cout << remainder;
    }
    return 0;
}