import sys,math,copy,queue,itertools,bisect
sys.setrecursionlimit(10**7)
LI = lambda : [int(x) for x in sys.stdin.readline().split()]
_LI = lambda : [int(x)-1 for x in sys.stdin.readline().split()]
NI = lambda : int(sys.stdin.readline())
INF = 10**18
MOD = 10**9 + 7
DD = ((1,0),(0,1),(-1,0),(0,-1))

N,M = LI()
A = LI()
ans = N - sum(A)
if ans < 0:
    ans = -1
print (ans)