#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define fi first
#define se second
#define pb push_back
#define sz(a) (ll) a.size()
#define all(x) (x).begin(), (x).end()
//[NOISG 2023 Finals] Airplane
ll n, m;
vector<ll> a;
void f() {
cin >> n >> m;
a.resize(n);
for(auto& u : a) cin >> u;
ll mxa=*max_element(all(a)), idmxa=max_element(all(a))-a.begin();
ll dummy;
for(ll i=0; i<(n-1)*2; i++) cin >> dummy;
ll ans=0, curr=0;
for(ll i=1; i<=idmxa; i++){
ans++;
curr++;
ans+=max(0LL, a[i]-curr);
curr=a[i];
}
curr=0;
for(ll i=n-2; i>=idmxa; i--){
ans++;
curr++;
ans+=max(0LL, a[i]-curr);
curr=a[i];
}
cout << ans;
}
int main() {
int tc=1;
// cin >> tc;
for(int i=1; i<=tc; i++){
// cout << '#' << i << endl;
f();
if(i!=tc) cout << endl;
}
}
