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#include <bits/stdc++.h>
using namespace std;
const int MAX = 2e5 + 100;
const int maxn = 50;
int n, ans;
bool allOut;
string s, t;
int degin[maxn], degout[maxn];
int G[maxn][maxn];
int fa[maxn], cnt[maxn];
bool vis[maxn];

void init() {
    for (int i = 0; i < 26; ++i) {
        fa[i] = i;
        cnt[i] = 1;
    }
}

int findF(int x) {
    return x == fa[x] ? x : fa[x] = findF(fa[x]);
}

void union_(int x, int y) {
    x = findF(x);
    y = findF(y);
    if (x != y) {
        fa[x] = y;
        degout[y] += degout[x];
        cnt[y] += cnt[x];
    }
}

int id(char ch) {
    return ch - 'a';
}

int main() {
#ifdef ExRoc
    freopen("test.txt", "r", stdin);
#endif // ExRoc

    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    cin >> n >> s >> t;
    for (int i = 0; i < n; ++i) {
        G[id(s[i])][id(t[i])] = 1;
        vis[id(s[i])] = true;
        vis[id(t[i])] = true;
    }
    bool allVis = true;
    for (int i = 0; i < 26; ++i) {
        if (!vis[i]) {
            allVis = false;
        }
        for (int j = 0; j < 26; ++j) {
            if (G[i][j] == 1) {
                ++degout[i];
                if (degout[i] > 1) {
                    cout << -1 << endl;
                    return 0;
                }
            }
        }
    }
    for (int i = 0; i < 26; ++i) {
        if (G[i][i] == 1) {
            --degout[i];
        }
    }
    init();
    for (int i = 0; i < 26; ++i) {
        for (int j = 0; j < 26; ++j) {
            if (G[i][j] == 1) {
                union_(i, j);
            }
        }
    }
    int circle = 0;
    memset(vis, 0, sizeof(vis));
    for (int i = 0; i < 26; ++i) {
        int f = findF(i);
        if (!vis[f]) {
            vis[f] = true;
            if (cnt[f] == degout[f]) {
                ans += degout[f] + 1;
                ++circle;
            } else {
                ans += degout[f];
            }
        }
    }
    if (circle > 0 && allVis) {
        cout << -1 << endl;
        return 0;
    }
    cout << ans << endl;

    return 0;
}