import std.stdio, std.array, std.string, std.conv, std.algorithm;
import std.typecons, std.range, std.random, std.math, std.container;
import std.numeric, std.bigint, core.bitop;

void main() {
    auto s = readln.split;
    auto N = s[0].to!int;
    auto P = s[1].to!long;
    auto H = readln.split.map!(to!long).array;

    long[] peaks;
    int p;

    while (p < N-1 && H[p] >= H[p+1])
        ++p;
    peaks ~= p;

    if (p == N-1) {
        writeln(0);
        return;
    }


    bool up = true;
    while (p < N-1) {
        if (up && H[p] > H[p+1]) {
            peaks ~= p;
            up = false;
        } else if (!up && H[p] < H[p+1]) {
            peaks ~= p;
            up = true;
        }
        ++p;
    }

    if (up && H[$-1] >= H[$-2])
        peaks ~= N-1;


    long ans = 0;
    p = 1;
    while (p < peaks.length) {
        if (H[peaks[p]] - H[peaks[p-1]] < P)
            ans += H[peaks[p]] - H[peaks[p-1]];
        else
            ans += P;
        p += 2;
    }

    ans.writeln;
}