我有一个字符数组，该数组会生成一个随机的小写字母数组，该数组由用户输入。我的问题是，在生成随机字母数组之后，用户输入一对字符（两个字母），我的代码应检查该对是否在所生成的随机数组中。当它只检查一个字母时它工作正常，但是当我介绍第二个字母时它不起作用。我将不胜感激。

#include <ctime>
#include <iostream>
#include <cstdlib>

int main() {
    std::srand(std::time(NULL));
    int i,num;
    char letter1,letter2,ch,r;
    const char chars[]="abcdefghijklmnopqrstuvwxyz";


    std::cout << "How many letters do you want in your string? ";
    std::cin >> num;

    for (i=0; i<num; i++)
    {
        ch = chars[rand()%26];
        std::cout << ch;
    }

    std::cout << "\nWhat letter pair would you like to find? ";
    std::cin >> letter1 >> letter2;


    if ((chars[i] == letter1) && (chars[i+1]==letter2))
        std::cout << "It is in your string. ";
    else
        std::cout << "You do not have " << letter1 << letter2 << " in your string";
}