JSFiddle - React, Tailwind, and code Playground

JavaScript

function getCount(array, h) {
    var max = 0, sum = 0;
    for (var i = 0; i < array.length; ++i) {
        var item = array[i];
        if (item >= h) {
            break;
        }
        sum += Math.max(0, max - item);
        max = Math.max(max, item);
    }
    return sum;
}

alert(getCount([1, 5, 3, 2, 3, 4, 5, 1, 4, 6], 6));