#include <bits/stdc++.h>


#define MAX 500000

int main()
{
    int count;
    char str[MAX];
    std::bitset<MAX> charPresence[30];

    scanf("%d ", &count);
    //memset(str, 'a', MAX);

    fgets(str, MAX, stdin);

    for(int i = 0; i < count; i++)
        str[i] -= 'a';

    for(int i = 0; i < count; i++)
        charPresence[str[i]].set(i);

    int symLength = 0;
    std::bitset<MAX> valid;
    valid.set();
    for(int i = count - 1; i >= 0; i--)
    {
        char c = str[i];

        valid &= charPresence[c];
        valid <<= 1;

        if(valid[count - 1])
            symLength = count - i;
        
        if(!valid.any())
            break;
    }

    printf("%d", count - symLength - 1);


    return 0;
}