const fs = require('fs');

// Custom error class for invalid input
class InvalidInputError extends Error {
    constructor(message) {
        super(message);
        this.name = "InvalidInputError";
    }
}

function validateInput(inputText) {
    if (!/^[a-zA-Z]+$/.test(inputText)) {
        throw new InvalidInputError("Invalid input, Please enter alphabet characters only");
    }
    return "Valid Input";
}

function main() {
    fs.readFile('input.txt', 'utf8', (err, data) => {
        if (err) {
            if (err.code === 'ENOENT') {
                console.error("The file 'input.txt' was not found.");
            } else {
                console.error("An unexpected error occurred:", err);
            }
            return;
        }

        const inputText = data.trim();
        
        try {
            const result = validateInput(inputText);
            console.log(result);
        } catch (e) {
            if (e instanceof InvalidInputError) {
                console.error(e.message);
            } else {
                console.error("An unexpected error occurred:", e);
            }
        }
    });
}

main();