/+ dub.sdl:
    name "A"
    dependency "dcomp" version=">=0.3.2"
    lflags "-stack_size" "100000000"
+/

import std.algorithm, std.conv, std.range, std.stdio;

// import dcomp.scanner;
// import dcomp.algorithm;
// import dcomp.container.deque;

int main(string[] argv) {
    auto sc = new Scanner();
    int n;
    string s, t;
    sc.read(n, s, t);

    //first check
    if (s == t) {
        writeln(0);
        return 0;
    }
    if (s[0] != t[0]) {
        writeln(-1);
        return 0;
    }

    int[][26] pos;
    foreach (i; 1..n) {
        int id = s[i] - 'a';
        pos[id] ~= i;
    }
    int[2][] node;
    int ba = n;
    foreach_reverse (i; 1..n) {
        if (t[i-1] == t[i]) continue;
        int id = t[i] - 'a';
        int r = min(ba-1, i);
        //check t[i], search id, [1, r]
        int lt = binSearch!(i => pos[id][i] > r)(-1, pos[id].length.to!int)-1;
        if (lt == -1) {
            writeln(-1);
            return 0;
        }
        int nw = pos[id][lt];
        node ~= [i, nw];
        ba = nw;
    }
    reverse(node);

    int ma = 0;
    Deque!(int[2]) q; // value, pos
    q.insertBack([-(10^^9), -1]);
    foreach (int co, nd; node) {
        int i = nd[0];
        int x = nd[1];
        while (q.length && q.back[0] >= i-co) {
            q.removeBack;
        }
        q.insertBack([i-co, co]);
        int lt = binSearch!(i => q[i][0] >= x+1-co)(-1, q.length.to!int)-1;
        ma = max(ma, co-q[lt][1]);
    }
    writeln(ma+1);
    return 0;
}
/* IMPORT /Users/yosupo/Program/dcomp/source/dcomp/container/deque.d */
// module dcomp.container.deque;

struct Deque(T) {
    import core.exception : RangeError;
    import core.memory : GC;
    import std.range : ElementType, isInputRange;
    import std.traits : isImplicitlyConvertible;

    struct Payload {
        T *d;
        size_t st, length, cap;
        @property bool empty() const { return length == 0; }
        alias opDollar = length;
        ref inout(T) opIndex(size_t i) inout {
            version(assert) if (length <= i) throw new RangeError();
            return d[(st+i >= cap) ? (st+i-cap) : st+i];
        }
        private void expand() {
            import std.algorithm : max;
            assert(length == cap);
            auto nc = max(4L, 2*cap);
            T* nd = cast(T*)GC.malloc(nc * T.sizeof);
            foreach (i; 0..length) {
                nd[i] = this[i];
            }
            d = nd; st = 0; cap = nc;
        }
        void insertFront(T v) {
            if (length == cap) expand();
            if (st == 0) st += cap;
            st--; length++;
            this[0] = v; 
        }
        void insertBack(T v) {
            if (length == cap) expand();
            length++;
            this[length-1] = v; 
        }
        void removeFront() {
            assert(!empty, "Deque.removeFront: Deque is empty");        
            st++; length--;
            if (st == cap) st = 0;
        }
        void removeBack() {
            assert(!empty, "Deque.removeBack: Deque is empty");        
            length--;
        }        
    }
    struct RangeT(A) {
        alias T = typeof(*(A.p));
        alias E = typeof(A.p.d[0]);
        T *p;
        size_t a, b;
        @property bool empty() const { return b <= a; }
        @property size_t length() const { return b-a; }
        @property RangeT save() { return RangeT(p, a, b); }
        @property RangeT!(const A) save() const {
            return typeof(return)(p, a, b);
        }
        alias opDollar = length;
        @property ref inout(E) front() inout { return (*p)[a]; }
        @property ref inout(E) back() inout { return (*p)[b-1]; }
        void popFront() {
            version(assert) if (empty) throw new RangeError();
            a++;
        }
        void popBack() {
            version(assert) if (empty) throw new RangeError();
            b--;
        }
        ref inout(E) opIndex(size_t i) inout { return (*p)[i]; }
        RangeT opSlice() { return this.save; }
        RangeT opSlice(size_t i, size_t j) {
            version(assert) if (i > j || a + j > b) throw new RangeError();
            return typeof(return)(p, a+i, a+j);
        }
        RangeT!(const A) opSlice() const { return this.save; }
        RangeT!(const A) opSlice(size_t i, size_t j) const {
            version(assert) if (i > j || a + j > b) throw new RangeError();
            return typeof(return)(p, a+i, a+j);
        }
    }
    
    alias Range = RangeT!Deque;
    alias ConstRange = RangeT!(const Deque);
    alias ImmutableRange = RangeT!(immutable Deque);

    Payload *p;
    private void I() { if (!p) p = new Payload(); }
    private void C() const { assert(p, "this deque is not init"); }
    //some value
    this(U)(U[] values...) if (isImplicitlyConvertible!(U, T)) {I;
        p = new Payload();
        foreach (v; values) {
            insertBack(v);
        }
    }
    //range
    this(Range)(Range r)
    if (isInputRange!Range &&
    isImplicitlyConvertible!(ElementType!Range, T) &&
    !is(Range == T[])) {I;
        p = new Payload();
        foreach (v; r) {
            insertBack(v);
        }
    }
    
    @property bool empty() const { return (!p || p.empty); }
    @property size_t length() const { return (p ? p.length : 0); }
    alias opDollar = length;
    ref inout(T) opIndex(size_t i) inout {C; return (*p)[i]; }
    ref inout(T) front() inout {C; return (*p)[0]; }
    ref inout(T) back() inout {C; return (*p)[$-1]; }
    void insertFront(T v) {I; p.insertFront(v); }
    void insertBack(T v) {I; p.insertBack(v); }
    void removeFront() {C; p.removeFront(); }
    void removeBack() {C; p.removeBack(); }
    Range opSlice() {I; return Range(p, 0, length); }
}

unittest {
    import std.algorithm : equal;
    import std.range.primitives : isRandomAccessRange;
    import std.container.util : make;
    auto q = make!(Deque!int);
    assert(isRandomAccessRange!(typeof(q[])));

    //insert,remove
    assert(equal(q[], new int[](0)));
    q.insertBack(1);
    assert(equal(q[], [1]));
    q.insertBack(2);
    assert(equal(q[], [1, 2]));
    q.insertFront(3);
    assert(equal(q[], [3, 1, 2]) && q.front == 3);
    q.removeFront;
    assert(equal(q[], [1, 2]) && q.length == 2);
    q.insertBack(4);
    assert(equal(q[], [1, 2, 4]) && q.front == 1 && q.back == 4 && q[$-1] == 4);
    q.insertFront(5);
    assert(equal(q[], [5, 1, 2, 4]));

    //range
    assert(equal(q[][1..3], [1, 2]));
    assert(equal(q[][][][], q[]));
    //const range
    const auto rng = q[];
    assert(rng.front == 5 && rng.back == 4);
    
    //reference type
    auto q2 = q;
    q2.insertBack(6);
    q2.insertFront(7);
    assert(equal(q[], q2[]) && q.length == q2.length);

    //construct with make
    auto a = make!(Deque!int)(1, 2, 3);
    auto b = make!(Deque!int)([1, 2, 3]);
    assert(equal(a[], b[]));
}

unittest {
    Deque!int a;
    Deque!int b;
    a.insertFront(2);
    assert(b.length == 0);
}

unittest {
    import std.algorithm : equal;
    import std.range : iota;
    Deque!int a;
    foreach (i; 0..100) {
        a.insertBack(i);
    }
    assert(equal(a[], iota(100)));
}
/* IMPORT /Users/yosupo/Program/dcomp/source/dcomp/scanner.d */
// module dcomp.scanner;

class Scanner {
    import std.stdio : File, stdin;
    import std.conv : to;
    import std.range : front, popFront, array, ElementType;
    import std.array : split;
    import std.traits : isSomeChar, isStaticArray, isArray; 
    import std.algorithm : map;
    File f;
    this(File f = stdin) {
        this.f = f;
    }
    string[] buf;
    private bool succ() {
        while (!buf.length) {
            if (f.eof) return false;
            buf = f.readln.split;
        }
        return true;
    }
    private bool readSingle(T)(ref T x) {
        if (!succ()) return false;
        static if (isArray!T) {
            alias E = ElementType!T;
            static if (isSomeChar!E) {
                //string or char[10] etc
                x = buf.front;
                buf.popFront;
            } else {
                static if (isStaticArray!T) {
                    //static
                    assert(buf.length == T.length);
                }
                x = buf.map!(to!E).array;
                buf.length = 0;                
            }
        } else {
            x = buf.front.to!T;
            buf.popFront;            
        }
        return true;
    }
    int read(T, Args...)(ref T x, auto ref Args args) {
        if (!readSingle(x)) return 0;
        static if (args.length == 0) {
            return 1;
        } else {
            return 1 + read(args);
        }
    }
}

unittest {
    import std.path : buildPath;
    import std.file : tempDir;
    import std.algorithm : equal;
    import std.stdio : File;
    string fileName = buildPath(tempDir, "kyuridenanmaida.txt");
    auto fout = File(fileName, "w");
    fout.writeln("1 2 3");
    fout.writeln("ab cde");
    fout.writeln("1.0 1.0 2.0");
    fout.close;
    Scanner sc = new Scanner(File(fileName, "r"));
    int a;
    int[2] b;
    char[2] c;
    string d;
    double e;
    double[] f;
    sc.read(a, b, c, d, e, f);
    assert(a == 1);
    assert(equal(b[], [2, 3]));
    assert(equal(c[], "ab"));
    assert(equal(d, "cde"));
    assert(e == 1.0);
    assert(equal(f, [1.0, 2.0]));
}
/* IMPORT /Users/yosupo/Program/dcomp/source/dcomp/algorithm.d */
// module dcomp.algorithm;

//[0,0,0,...,1,1,1]で、初めて1となる場所を探す。pred(l) == 0, pred(r) == 1と仮定
T binSearch(alias pred, T)(T l, T r) {
    while (r-l > 1) {
        T md = (l+r)/2;
        if (!pred(md)) l = md;
        else r = md;
    }
    return r;
}

import std.range.primitives;

Rotator!Range rotator(Range)(Range r)
if (isForwardRange!Range && hasLength!Range) {
    return typeof(return)(r);
}

struct Rotator(Range)
if (isForwardRange!Range && hasLength!Range) {
    size_t cnt;
    Range start, now;
    this(Range r) {
        cnt = 0;
        start = r.save;
        now = r.save;
    }
    this(this) {
        start = start.save;
        now = now.save;
    }
    @property bool empty() {
        return now.empty;
    }
    @property auto front() {
        assert(!now.empty);
        import std.range : take, chain;
        return chain(now, start.take(cnt));
    }
    @property Rotator!Range save() {
        return this;
    }
    void popFront() {
        cnt++;
        now.popFront;
    }
}


E minimum(alias pred = "a < b", Range, E = ElementType!Range)(Range range, E seed)
if (isInputRange!Range && !isInfinite!Range && !is(CommonType!(ElementType!Range, E) == void)) {
    import std.functional;
    while (!range.empty) {
        if (binaryFun!pred(range.front, seed)) {
            seed = range.front;
        }
        range.popFront;
    }
    return seed;
}

ElementType!Range minimum(alias pred = "a < b", Range)(Range range)
if (isInputRange!Range && !isInfinite!Range) {
    assert(!range.empty, "range must not empty");
    auto e = range.front; range.popFront;
    return minimum!pred(range, e);
}

unittest {
    assert(minimum([2, 1, 3]) == 1);
    assert(minimum!"a > b"([2, 1, 3]) == 3);
    assert(minimum([2, 1, 3], -1) == -1);
    assert(minimum!"a > b"([2, 1, 3], 100) == 100);
}

bool[ElementType!Range] toMap(Range)(Range r) {
    import std.algorithm : each;
    bool[ElementType!Range] res;
    r.each!(a => res[a] = true);
    return res;
}


Measured memory_bytes: 22996