DevGod needs to write a blog entry for this problem!

/**
 * @param {string} s
 * @param {string} t
 * @return {number}
 */
var appendCharacters = function(s, t) {
    let L = 0;
    let R = 0;

    while(L<s.length){
        if(s[L] === t[R]){
            L++;
            R++;
        }else{
            L++;
        }
    }

    return t.length-R;
};