Auto Key Cipher Encryption

//auto_key_cipher
#include <iostream>
#include <cstdio>
#include <string>
using namespace std;
#define siz 100
int p[siz], c[siz], k[siz];

int main()
{
    int key, x;
    string plainText, cipherText;
    freopen("input.txt", "r", stdin);
    while(cin>>plainText>>key){
        //encryption
        int len = plainText.size();
        for(int i=0; i<len; i++){
            p[i] = plainText[i] - 'a';
            if(i==0) k[i] = key;
            else k[i] = p[i-1];
            c[i] = (p[i] + k[i]) % 26;
            cipherText += c[i] + 'A';
        }
        cout<<"Cipher Text: "<<cipherText<<endl;
    }
    return 0;
}