## https://yukicoder.me/problems/no/2217


def solve(A, K, value):
    x = 0

    for i in range(len(A)):

        a = A[i]
        if a + x >= value:
            continue
        y = value - (a + x)

        z = y // (i + 1)
        if y % (i + 1) != 0:
            z += 1
        
        if K - z < 0:
            return False
        K -= z
        x += z * (i + 1)
    return True


def main():
    N, K = map(int, input().split())
    A = list(map(int, input().split()))

    low = 0
    high = 10 ** 16
    while high - low > 1:
        mid = (high  + low ) // 2
        if solve(A, K, mid):
            low = mid
        else:
            high = mid
    if solve(A, K, high):
        print(high)
    else:
        print(low)






if __name__ == "__main__":
    main()