import copy
S=input()
T=input()
nexS=[[0] * 26 for i in range(100005)]
nexT=copy.deepcopy(nexS)

for i in range(26):
    nexS[len(S)][i]=len(S)
    nexT[len(T)][i]=len(T)
for i in range(len(S)-1,-1,-1):
    for j in range(26):
        nexS[i][j]=nexS[i+1][j]
    nexS[i][ord(S[i])-0x61]=i
for i in range(len(T)-1,-1,-1):
    for j in range(26):
        nexT[i][j]=nexT[i+1][j]
    nexT[i][ord(T[i])-0x61]=i

inc=[0 for i in range(100005)]
now=len(T)
inc[len(S)]=now
for i in range(len(S)-1,-1,-1):
    now=max(-1,now-1)
    while now>=0 and T[now]!=S[i]:
        now-=1
    inc[i]=now
if inc[0]!=-1:
    print(-1)
    exit(0)

Ans=""
curS,curT=0,0

while curS<len(S):
    for i in range(26):
        if nexS[curS][i]==len(S):
            continue
        if nexT[curT][i]==len(T):
            Ans+=(chr)(0x61+i)
            print(Ans)
            exit(0)
        if inc[nexS[curS][i]+1]>=nexT[curT][i]+1:
            continue
        curS=nexS[curS][i]+1
        curT=nexT[curT][i]+1
        Ans+=(chr)(0x61+i)
        break