551C - GukiZ hates Boxes - CodeForces Solution

f = lambda: map(int, input().split())
n, m = f()
h = list(f())[::-1]
def g(t, m):
    t -= n
    d = 0
    for u in h:
        t+=1
        if u>d:
            if t<1: 
                return 1
            u-=d
            d=-u % t
            m-=(u + d) // t
            if m<0: return 1
        else:
            d-=u
    return 0
a,b=0,int(11e13)
while b - a > 1:
    c=(b+a)//2
    if g(c,m):
        a=c
    else:
        b=c
print(b+1)