from __future__ import print_function
[N, M] = map(int, raw_input().split())
d = [-1 for i in range(N)]
for i in range(M):
    [a,b] = map(int, raw_input().split())
    if d[a-1] == -1:
        d[a-1] = b
    elif d[a-1] != b:
        print("-1")
        exit(0)

if d[0] == -1:
    #print 1,
    print("1",end="")
elif d[0] == 0 and N > 1:
    print ("-1")
    exit(0)
elif N > 1:
    #print d[0],
    print(d[0], end="")

for i in range(1,N-1):
        if d[i] == -1:
            #print 0,
            print("0",end="")
        else:
            #print d[i],
            print(d[i], end="")

if d[N-1] == -1:
    #print 0
    print(0)
else:
    #print d[N-1]
    print(d[N-1])
