#include<bits/stdc++.h>
 
using namespace std;
 
#define pii pair<int,int>
#define ll long long
#define N (int)(1e5+10)
#define mod 1000000007
#define mp make_pair
#define pb push_back
#define nd second
#define st first
#define inf mod
#define endl '\n'
#define sag (sol|1)
#define sol (root<<1)
#define ort ((bas+son)>>1)
#define bit(x,y) ((x>>y)&1)
 
int main(){
	int H[500]={0};
	int t=0,i,n;
	char str[N];
 
	cin >> n;
 
	scanf("%s",str);
 
	for(i=0 ; i<n ; i++)
		t += (H[str[i]]++) == 0;
 
	if(n > 26)
		puts("-1");
	else
		cout << n - t << endl;
}
 

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