题解 | #神奇的字符#

from collections import namedtuple
import sys

data  = []
for line in sys.stdin:
    data.append(line.split())
# print(data)

n = int(data[0][0])
m = int(data[0][1])
S = data[1][0]
T = data[2][0]

if n < m :
    print(-1)

t,s = 0,0

while(t < m and s < n):
    if(T[t] == S[s]):
        t += 1
        s += 1
    else:
        s += 1

if(t != m):
    print(-1)
else:
    print(s)