// Copyright (C) 2017 __debug.

// This program is free software; you can redistribute it and/or
// modify it under the terms of the GNU General Public License as
// published by the Free Software Foundation; version 3

// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// You should have received a copy of the GNU General Public License
// along with this program; If not, see <http://www.gnu.org/licenses/>.


#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/priority_queue.hpp>

#define x first
#define y second
#define MP std::make_pair
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(), (x).end()
#define DEBUG(...) fprintf(stderr, __VA_ARGS__)
#ifdef __linux__
#define getchar getchar_unlocked
#define putchar putchar_unlocked
#endif

using std::pair;
using std::vector;
using std::string;

typedef long long LL;
typedef pair<int, int> Pii;

const int oo = 0x3f3f3f3f;

template<typename T> inline bool chkmax(T &a, T b) { return a < b ? a = b, true : false; }
template<typename T> inline bool chkmin(T &a, T b) { return b < a ? a = b, true : false; }
string procStatus()
{
    std::ifstream t("/proc/self/status");
    return string(std::istreambuf_iterator<char>(t), std::istreambuf_iterator<char>());
}
template<typename T> T read(T &x)
{
    int f = 1;
    char ch = getchar();
    for (; !isdigit(ch); ch = getchar())
        f = (ch == '-' ? -1 : 1);
    for (x = 0; isdigit(ch); ch = getchar())
        x = 10 * x + ch - '0';
    return x *= f;
}
template<typename T> void write(T x)
{
    if (x == 0) {
        putchar('0');
        return;
    }
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    static char s[20];
    int top = 0;
    for (; x; x /= 10)
        s[++top] = x % 10 + '0';
    while (top)
        putchar(s[top--]);
}
// EOT

const int MAXN = 1e6 + 5;

int N;
char S[MAXN], T[MAXN];

void input()
{
    read(N);
    scanf("%s", S + 1);
    scanf("%s", T + 1);
}

void solve()
{
    if (strcmp(S + 1, T + 1) == 0) {
        puts("0");
        return;
    }

    static Pii q[MAXN];
    int front = 0, rear = 0;
    int offset = 0, add = 0;
    int ans = 0;

    q[rear++] = MP(N + 1, 233);
    for (int i = N; i >= 1; --i) {
        if (T[i] != T[i - 1]) {
            while (front < rear && q[front].x + offset > i) {
                ++front;
            }
            int j = front < rear ? q[rear - 1].x + offset - 1 : i;
            while (j > 0 && S[j] != T[i]) {
                --j;
            }
            if (j == 0) {
                puts("-1");
                return;
            }
            if (i == j)
                continue;
            --offset; ++add;
            if (q[rear - 1].x + offset > j)
                q[rear++] = MP(j - offset, 1 - add);
            chkmax(ans, q[front].y + add);
        }
    }
    ++ans;

    printf("%d\n", ans);
}

int main()
{
#ifdef __DEBUG
    freopen("F.in", "r", stdin);
    freopen("F.out", "w", stdout);
#endif

    input();
    solve();

    return 0;
}

// 相思了无益，悔当初相见。
//     -- 朱彝尊《忆少年·飞花时节》


Measured memory_bytes: 4352