import sys

r = int(sys.stdin.readline().strip())
unit = int(sys.stdin.readline().strip())
n = int(sys.stdin.readline().strip())

if n == 0:
    print(-1)
    sys.exit()

arr = list(map(int, sys.stdin.readline().strip().split()))

req = r * unit
food = 0

for i in range(n):
    food += arr[i]
    if food >= req:
        print(i + 1)
        sys.exit()

print(0)

convertipv6toipv4(ipv6addr)