【CodeForces 672B】Different is Good

#include<cstdio>
char s;
int n,u[30],ans;
int main(){
scanf("%d ",&n);
for(int i=0;i<n;i++){
scanf("%c ",&s);
u[s-'a']++;
}
if(n>26)printf("-1");
else{
for(int i=0;i<26;i++)
if(u[i])ans+=u[i]-1;
printf("%d",ans);
}
}