#include <bits/stdc++.h>
using namespace std;
vector<int> des[1000010];
int col[1000010], sum[1000010], fa[1000010], st[1000010], ff[1000010];
bool ins[1000010];
int getf(int s) {
  if (ff[s] == s) return s;
  return ff[s] = getf(ff[s]);
}
void dfs(int s, int pre) {
  col[s] = -col[pre];
  sum[s] = col[s];
  for (int k = 0; k < des[s].size(); k++)
    if (des[s][k] != pre) {
      fa[des[s][k]] = s;
      dfs(des[s][k], s);
      sum[s] += sum[des[s][k]];
    }
}
int main() {
  int n, m, x = 1, y = 0;
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; i++) ff[i] = i;
  for (int i = 1; i <= m; i++) {
    int u, v;
    scanf("%d%d", &u, &v);
    if (getf(u) == getf(v)) {
      x = u;
      y = v;
      break;
    }
    ff[getf(u)] = getf(v);
    des[u].push_back(v);
    des[v].push_back(u);
  }
  col[0] = -1;
  dfs(x, 0);
  if (!y) {
    if (sum[x]) {
      printf("-1\n");
      return 0;
    }
    long long ans = 0;
    for (int i = 1; i <= n; i++) ans += abs(sum[i]);
    printf("%lld\n", ans);
    return 0;
  }
  int top = 0;
  for (int i = y; i != 0; i = fa[i]) {
    ins[i] = true;
    st[++top] = i;
  }
  if (top & 1) {
    if (sum[x] & 1) {
      printf("-1\n");
      return 0;
    }
    long long ans = 0;
    for (int i = 1; i <= n; i++)
      if (ins[i])
        ans += abs(sum[i] - sum[x] / 2);
      else
        ans += abs(sum[i]);
    printf("%lld\n", ans);
    return 0;
  }
  if (sum[1]) {
    printf("-1\n");
    return 0;
  }
  sort(st + 1, st + top + 1);
  long long ans = 0;
  for (int i = 1; i <= n; i++)
    if (ins[i])
      ans += abs(sum[i] - st[top / 2]);
    else
      ans += abs(sum[i]);
  printf("%lld\n", ans);
}
