k, n = list(map(int, input().split())) A = list(map(int, input().split())) t = s = 0 x = sum(A)-k for a in A: x -= a-1 l = t % n r = (t+a-1) % n a0 = l if l else n a1 = n-r ans = 1<<60 if a1 <= x: ans = min(ans, a1) if a0 <= s: ans = min(ans, a0) if ans == 1<<60: print(-1) else: print(ans) #print(s, x, a0, a1) t += a s += a-1