675 B | 675 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from decimal import Decimal
BLANK = " "
inp = input
# inp = lambda : sys.stdin.readline()[:-1].strip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def solve():
n, k = mii()
l = mii()
cnt = 0
for i in range(1, n):
if l[i - 1] < l[i]:
continue
l[i] += k
cnt += 1
for i in range(1, n):
if l[i - 1] < l[i]:
continue
print(-1)
return
print(cnt)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()