847M - Weather Tomorrow - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
#define ll long long int
int main(){
ll n; cin>>n;
ll a[n];
for(ll i=0;i<n;i++){
    cin>>a[i];
}
ll d=a[1]-a[0];
for(ll i=2;i<n;i++){
    if(a[i]-a[i-1]!=d){
        cout<<a[n-1]; return 0;
    }
}
cout<<d+a[n-1];
}