#include <stdio.h>
int main(){
    FILE*fin = fopen("input.txt","r");
    FILE*fout = fopen("output.txt","w");
    int N;
    fscanf(fin,"%i",&N);
    int digits[100], position [100], arifmetical[100];
    for (int  i = 0;  i < N; ++ i) {
        fscanf(fin,"%i",&digits[i]);
    }
    int a = 1;
    arifmetical[1] = 1;
    for (int i = 0; i < N; ++i) {
        for (int j = i+1; j < N; ++j) {
            int b = 2;
            int e=j;
            position[1] = i;
            position[2] = j;
            int p =digits[i]*(1-b)+digits[j]*b;
            int d = 0;
            for (int k = e+1; k < N; ++k) {
                if (digits[k]==p){
                    d = k;
                    break;
                }
                if (d == 0){
                    break;
                }else{
                    b+=1;
                    position[d]=d;
                    e = d;
                }
            }
            if (b>a){
                a = b;
                for (int k = 0; k < b; ++k) {
                    arifmetical[k] = position [k];
                }
            }
        }
    }
    fprintf(fout,"%i\n",a);
    for (int l = 0; l < a; ++l) {
        fprintf(fout,"%i ",arifmetical[l]);
    }
    fclose(fin);
    fclose(fout);
    return 0;


}