P5

//Problema 5
#include <stdio.h>
int v[100];
int find_missing(int v[], int l, int r, int ratio){

        if(l <= r){
                int mid = (l+r)/2;
                if(v[mid + 1] - v[mid] != ratio)
                    return v[mid]+ratio;
                if(mid > 0 && v[mid] - v[mid-1] != ratio)
                    return v[mid-1] + ratio;
                if(v[mid] == v[0] + ratio*mid)
                    return find_missing(v,mid+1,r,ratio);
                else
                    return find_missing(v,l,mid-1,ratio);
        }

}
int find_ratio(int v[], int n){
    int ratio = (v[n-1] - v[0])/n;
    return ratio;
}

int main(){
    int n;
    scanf("%d", &n);
    for(int i=0; i<n;i++)
        scanf("%d",&v[i]);
    int ratio = find_ratio(v,n);
    printf("%d\n",find_missing(v,0,n-1,ratio));
    return 0;
}