import sys
n,m = map(int,input().split())
s = [0 for _ in range(m)]
c = [0 for _ in range(m)]
for i in range(m):
    s[i],c[i] = map(int,input().split())
if(max(s) < n):
    print(-1)
    sys.exit()
else:
    e = [9 for _ in range(n)]
    flag = [0 for _ in range(n)]

    for j in range(m):
        if(flag[s[j]-1] == 0):
            e[s[j]-1] = c[j]
        else:
            e[s[j]-1] = min(c[j],e[s[j]-1])
        flag[s[j]-1] = 1
    if(e[0] == 0):
        print(-1)
        sys.exit()
    for k in range(n):
        if(flag[k] == 0):
            e[k] = 0
    e = map(str,e)
    print("".join(e))