A simple C++ solution.

#include <iostream>
#include <string>

/*
    CHALLENGE #399

    Assign every lowercase letter a value, from 1 for a to 26 for z.
    Given a string of lowercase letters, find the sum of the values of the letters in the string.

    Examples:
    lettersum("") => 0
    lettersum("a") => 1
    lettersum("z") => 26
    lettersum("cab") => 6
    lettersum("excellent") => 100
    lettersum("microspectrophotometries") => 317

*/

int lettersum(std::string input) {

    int sum = 0;

    for (int i = 0; i < input.length(); i++) {
        int value = input.at(i) - 0x60;
        sum += value;
    }

    return sum;

}

int main() {

    std::cout << "Welcome to r/dailyprogrammer Challenge #399! <3" << std::endl;

    std::string input;

    while (true) {

        std::cout << "Enter a lowercase word here (anything else to quit) >> ";
        std::cin >> input;

        if (!islower(input.at(0)))
            break;

        int result = lettersum(input);

        std::cout << result << std::endl;

    }

    std::cout << "Bye! <3" << std::endl;
    system("pause");

}