#include <bits/stdc++.h>
long long read() {
  long long x = 0;
  char f = 1, ch = getchar();
  while (ch < '0' || ch > '9') {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (ch >= '0' && ch <= '9') {
    x = x * 10 + ch - '0';
    ch = getchar();
  }
  return x * f;
}
void write(long long x) {
  if (x < 0) x = -x, putchar('-');
  if (x > 9) write(x / 10);
  putchar(x % 10 + '0');
}
inline void writeln(long long x) {
  write(x);
  puts("");
}
using namespace std;
const int N = 1100, M = 210000;
int ver[M], nxt[M], head[N], en;
inline void add(int x, int y) {
  ver[++en] = y, nxt[en] = head[x], head[x] = en;
  ver[++en] = x, nxt[en] = head[y], head[y] = en;
}
int n, m, d[N], bel[N], tot;
bool Fg;
void dfs(int x, int F) {
  d[x] = d[F] + 1;
  bel[x] = tot;
  for (int i = head[x]; i; i = nxt[i]) {
    int y = ver[i];
    if (y == F) continue;
    if (!d[y])
      dfs(y, x);
    else if ((d[x] - d[y] + 1) & 1)
      Fg = true;
  }
}
int q[N], l, r, f[N], mx[N];
long long ans;
int main() {
  n = read(), m = read();
  for (int i = 1; i <= m; ++i) add(read(), read());
  for (int i = 1; i <= n; ++i)
    if (!d[i]) {
      ++tot;
      dfs(i, 0);
      if (Fg) {
        puts("-1");
        return 0;
      }
    }
  for (int i = 1; i <= n; ++i) {
    memset(f, 0, sizeof f);
    l = 0, r = 1;
    q[++l] = i;
    f[i] = 1;
    while (l <= r) {
      int x = q[l++];
      for (int j = head[x]; j; j = nxt[j])
        if (!f[ver[j]]) {
          f[ver[j]] = f[x] + 1;
          q[++r] = ver[j];
        }
    }
    for (int j = 1; j <= n; ++j) mx[bel[i]] = max(mx[bel[i]], f[j]);
  }
  for (int i = 1; i <= tot; ++i) ans += mx[i] - 1;
  writeln(ans);
  return 0;
}
