import java.util.Scanner;
public class Main {
static class UnionFind {
private int[] parent;
private int[] rank;
public UnionFind(int n) {
parent = new int[n];
rank = new int[n];
for (int i = 0; i < n; i++) {
parent[i] = i;
rank[i] = 1;
}
}
public int find(int x) {
if (parent[x] != x) {
parent[x] = find(parent[x]);
}
return parent[x];
}
public boolean union(int x, int y) {
int rootX = find(x);
int rootY = find(y);
if (rootX != rootY) {
if (rank[rootX] > rank[rootY]) {
parent[rootY] = rootX;
} else if (rank[rootX] < rank[rootY]) {
parent[rootX] = rootY;
} else {
parent[rootY] = rootX;
rank[rootX]++;
}
return true;
}
return false;
}
}
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int m = scanner.nextInt();
UnionFind uf = new UnionFind(n);
int extraCables = 0;
for (int i = 0; i < m; i++) {
int a = scanner.nextInt();
int b = scanner.nextInt();
if (!uf.union(a, b)) {
extraCables++;
}
}
int connectedComponents = 0;
for (int i = 0; i < n; i++) {
if (uf.find(i) == i) {
connectedComponents++;
}
}
int requiredCables = connectedComponents - 1;
if (extraCables >= requiredCables) {
System.out.println(requiredCables);
} else {
System.out.println(-1);
}
scanner.close();
}
}