#include<cstdio>
int t[20];

int main(){
    int n;
    scanf("%d",&n);
    for(int i = 1; i <= n; i++) scanf("%d",&t[i]);
    int ans = -1, cnt = 0;
    int offset[7] = {0,2,4,5,7,9,11};
    for(int i = 0; i < 12; i++){
        int ok = 1;
        for(int j = 1; j <= n; j++){
            int find = 0;
            for(int k = 0; k < 7; k++) if((i+offset[k])%12==t[j]) find = 1;
            if(!find) ok = 0;
        }
        if(ok){
            ans = i;
            cnt++;
        }
    }
    if(cnt==1) printf("%d\n",ans);
    else printf("-1\n");
    return 0;
}