给定代码中①处应填什么？

#include <bits/stdc++.h>
using namespace std;
const int N = 510;
int n, c[N], f[N][N];
int main(){
    scanf("%d", &n);
    for (int i = 1; i <= n; i++) scanf("%d", &c[i]);
    memset(f, 0x3f, sizeof f);
    for (int len = 1; len <= n; len++)
        for (int l = 1; l + len - 1 <= n; l++){
            int r = l + len - 1;
            if (len == 1) f[l][r] = 1;
            else if (len == 2){
                if (c[l] == c[r]) f[l][r] = 2;
                else f[l][r] = 3;
            }
            else{
                if (c[l] == c[r]) f[l][r] = 4;
                for (int k = l; k < r; k++)
                    f[l][r] = min(f[l][r], 5);
            }
        }
    printf("%d\n", f[1][n]);
    return 0;
}