#include <bits/stdc++.h>   using namespace std;   using ll = long long;   using ld = long double;   ll x[1 << 18];   int main() {     ios::sync_with_stdio(false), cin.tie(0);     ll n, r;     cin >> n >> r;     ll m = 1 << n;     ll sum = 0;     for (int i = 0; i < m; i++) cin >> x[i], sum += x[i];     cout << fixed << setprecision(7);     cout << (ld)sum / (ld)m << endl;     for (int i = 0; i < r; i++) {       ll a, b;       cin >> a >> b;       sum -= x[a];       x[a] = b;       sum += x[a];       cout << (ld)sum / (ld)m << endl;     }   }   