暴力穷举

#include <iostream>
using namespace std;

int n, l, r;

int main(){
    cin >> n >> l >> r;
    int ans = -1;
    for(int i = l; i <= r; i++){
        ans = max(ans, i % n);
    }
    cout << ans;
    return 0;
}

数学方法

#include <iostream>
using namespace std;

int n, l, r;

int main(){
    cin >> n >> l >> r;
    if(l/n == r/n) cout << max(l%n, r%n);
    else cout << n-1;
    return 0;
}