#include <stdio.h>
#include <algorithm>
using namespace std;
const int MAXINT = 2000000000;
const int N = 1005;
struct Node {int a, b, c;}A[N*N], B[N*N];
int n;
int x[N];

bool operator<(const Node& a, const Node& b) {
	return a.c < b.c;
}

int main() {
	int i, j;
	while(scanf("%d", &n), n) {
		if(n < 4) {
			printf("no solution\n");
			continue;
		}
		for(i = 0; i < n; i++)
			scanf("%d", x+i);
		int k = 0;
		for(i = 0; i < n; i++) {
			for(j = i+1; j < n; j++) {
				if(x[i] != x[j]) {
					A[k].c = x[i] + x[j];
					A[k].a = x[i];
					A[k].b = x[j];
					k++;
				}
			}
		}
		int k2 = 0;
		for(i = 0; i < n; i++) {
			for(j = 0; j<n; j++) {
				if(x[i] != x[j]) {
					B[k2].c = x[i] - x[j];
					B[k2].a = x[i];
					B[k2].b = x[j];
					k2++;
				}
			}
		}
		sort(A, A+k);
		sort(B, B+k2);
		i = k-1, j = k2-1;
		int _max = -MAXINT;
		while(i >= 0 && j >= 0) {
			if(A[i].c != B[j].c) {
				if(A[i].c > B[j].c) i--;
				else j--;
				continue;
			}
			int o = j;
			while(A[i].c == B[j].c && j >= 0) {
				if(A[i].a != B[j].a && A[i].a != B[j].b && A[i].b != B[j].a && A[i].b != B[j].b) {
					if(B[j].a > _max) _max = B[j].a;
				}
				j--;
			}
			i--;
			j = o;
		}
		if(_max == -MAXINT) printf("no solution\n");
		else printf("%d\n", _max);
	}
	return 0;
}

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