import sys
N,M = map(int,input().split())
sc = [list(map(int,input().split())) for i in range(M)]

ans = [0] * N
checkd = [False] * N

for i in range(M):
    if not checkd[sc[i][0]-1]:
        checkd[sc[i][0]-1] = True
        ans[sc[i][0]-1] = sc[i][1]
    else:
        if ans[sc[i][0]-1] != sc[i][1]:
            print('-1')
            sys.exit()

if N >= 2 and checkd[0] and ans[0] == 0:
    print('-1')
    sys.exit()

real = 0
for i in ans:
    real = real * 10 + i
    
print(real)