为啥全红啊，大佬们帮忙看看蒟蒻的代码

#include<iostream>
#include<string>
using namespace std;
int main(){
    string str;
    char c[15],num[9];
    cin >> str;
    str.copy(c,13,0);
    *(c+13) = '\0';
    int j = 0,k = 0;
    while(c[k] != '\0'){
        if(c[k] != '-'){
            num[j] = c[k];
            j++;
            k++;
        }
        else
        k++;
    }
    int a[10], sum;
    for(int i =0;i < 9;i++)
        a[i] = num[i] - '0';
    for(int i =0;i < 9;i++)
        sum = sum + a[i] * ( i + 1 );
    sum = sum % 11;
    if(sum != 10){
        if(sum == c[12] - '0')
            cout << "Right";
        else{
            for(int i = 0;i < 12;i++)
            cout << c[i];
        cout << sum ;}
        }
    else
        {
            if(c[12] == 'X')
                cout << "Right";
            else{
                for(int i = 0;i < 12;i++)
                cout << c[i];
            cout << 'X';}
        }
    return 0;
}