#include<bits/stdc++.h>
#define L(i, j, k) for(int i = (j); i <= (k); ++i)
#define R(i, j, k) for(int i = (j); i >= (k); --i)
#define ll long long 
#define vi vector < int > 
#define sz(a) ((int) (a).size())
#define ll long long 
#define ull unsigned long long
#define me(a, x) memset(a, x, sizeof(a)) 
using namespace std;
const int N = 1 << 20;
int n, m;
vi e[N];

int ind[N], ord[N], top;
int dp[N], lst[N];

bool ask(int x, int y) {
	cout << "? " << x << ' ' << y << endl;
	int w;
	cin >> w;
	return w;
}

int main() {
	ios :: sync_with_stdio(false);
	cin.tie(0); cout.tie(0);
	cin >> n >> m;
	L(i, 1, m) {
		int u, v;
		cin >> u >> v;
		e[u].emplace_back(v);
		ind[v] += 1;
	}
	queue < int > q;
	L(i, 1, n) {
		if(ind[i] == 0) {
			q.push(i);
		}
	} 
	while(!q.empty()) {
		int u = q.front();
		q.pop(); 
		ord[++top] = u;
		for(auto v : e[u]) {
			--ind[v];
			if(!ind[v]) {
				q.push(v);
			}
		}
	}
	
	while(true) {
		L(i, 1, n) dp[i] = lst[i] = 0;
		L(o, 1, top) {
			int u = ord[o];
			for(auto&v : e[u]) {
				if(v && dp[v] < dp[u] + 1) {
					dp[v] = dp[u] + 1;
					lst[v] = u;
				}
			}
		}
		int p = max_element(dp + 1, dp + n + 1) - dp;
		vi R;
		while(true) {
			R.emplace_back(p);
			if(!dp[p]) break;
			p = lst[p];
		} 
		reverse(R.begin(), R.end());
		int win = 1;
		L(i, 1, sz(R) - 1) {
			if(!ask(R[i - 1], R[i])) {
				win = 0;
				for(auto&v : e[R[i - 1]]) if(v == R[i]) v = 0;
				break;
			}
		}
		if(win) {
			cout << "! " << *max_element(dp + 1, dp + n + 1) << '\n';
			return 0;
		}
	}
	return 0;
}