na982

#include <stdio.h>

int n, m, h, ret;
int map[31][11];

bool check() {
	bool ret = true;

	for (int i = 1; i <= n; ++i) {
		int pos = i;

		for (int j = 0; j <= h; ++j) {
			if (map[j][pos] == 1) {
				++pos;
			}
			else if (map[j][pos - 1] == 1) {
				--pos;
			}
		}

		if (pos != i) {
			return ret = false;
		}
	}

	return ret;
}

void dfs(int count, int y, int x) {
	if (count >= ret)	return;
	if (check()) {
		ret = count;
		return;
	}
	if (count == 3)	return;
	for (int i = y; i <= h; ++i) {
		for (int j = x; j < n; ++j) {
			if (map[i][j] == 0 && map[i][j - 1] == 0 && map[i][j + 1] == 0) {
				map[i][j] = 1;
				dfs(count + 1, i, j);
				map[i][j] = 0;
			}
		}
		x = 1;
	}
}

int main()
{
	scanf("%d %d %d", &n, &m, &h);
	int a, b;
	for (int i = 0; i < m; ++i) {
		scanf("%d %d", &a, &b);
		map[a][b] = 1;
	}

	ret = 4;
	dfs(0, 1, 1);
	if (ret == 4)	ret = -1;
	printf("%d\n", ret);

	return 0;
}