N, P = map(int, input().split())
a = list(map(int, input().split()))
cost = 0
inc = 0
up = 0
warp = 0
for n, m in zip(a, a[1:]):
    if n <= m:
        if inc == 0:
            warp += P
        inc = 1
        up += m-n
    else:
        if inc == 1:
            if warp > up:
                warp = up
            warp += P
        
print(min(up, warp))