import sys
N,M = map(int,input().split())
s = [0]*M
c = [0]*M
ans = [0]*N
for i in range(M):
    s[i],c[i] = map(int,input().split())
if N != 1:
    for i in range(M):
        if s[i] == 1 and c[i] == 0:
            print(-1)
            sys.exit()
for i in range(M):
    for j in range(i+1,M):
        if s[i] == s[j] and c[i] != c[j]:
            print(-1)
            sys.exit()
for i in range(M):
    ans[s[i]-1] = c[i]
for i in range(N):
    print(ans[i], end="")
