---

import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int n = Integer.parseInt(br.readLine().trim());
        String s = br.readLine().trim();
        String t = br.readLine().trim();

        List<Integer> diffs = new ArrayList<>();
        for (int i = 0; i < n; i++) {
            if (s.charAt(i) != t.charAt(i)) {
                diffs.add(i + 1); // По условию нумерация с 1
            }
        }

        if (diffs.size() % 2 != 0) {
            System.out.println("NO SOLUTION");
        } else {
            StringBuilder sb = new StringBuilder();
            sb.append(diffs.size() / 2).append("\n");
            for (int i = 0; i < diffs.size(); i += 2) {
                sb.append(diffs.get(i)).append(" ").append(diffs.get(i + 1)).append("\n");
            }
            System.out.print(sb);
        }
    }
}