找到规律后如下，只需要用高精度加法和四舍五入（本质也是高精度加法就能做），如果没有找到规律，就得自己写高精度乘法和加法，不熟练很容易错。

//#include<bits/stdc++.h>
#include<iostream>
#include<math.h>
using namespace std;

#define int long long
#define endl '\n'

string d;
int n;
//高精度加
string my_add(string a){
    int len = a.size(),x = 0;
    string temp = a;
    reverse(a.begin(),a.end());
    reverse(temp.begin(),temp.end());
    for(int i=0;i<a.size();i++){
        int y = temp[i]-'0'+a[i]-'0'+x;
        if(y<=9){
            a[i]=y+'0';
            x=0;
        }else{
            a[i]=y%10+'0';
            y/=10,x=y;
        }
    }
    if(x)a.append(1,x+'0');
    reverse(a.begin(),a.end());
    return a;
}
//四舍五入
string my_round(string ans){
    unsigned long index = ans.find('.');
    char ch = ans[index+1];
    ans = ans.erase(index);
    if(ch-'0'>=5){
        reverse(ans.begin(),ans.end());
        unsigned long len = ans.size();
        int x = 1;
        for(int i=0;i<len;i++){
            int y = ans[i]-'0'+x;
            if(y<=9){
                ans[i]=y+'0';
                x=0;
            }else{
                ans[i]=y%10+'0';
                y/=10,x=y;
            }
        }
        if(x)ans.append(to_string(x));
        reverse(ans.begin(),ans.end());
        return ans;
    }else return ans;
}

void solve(){
    cin>>n>>d;
    int len = d.size();
    if(d[len-1]=='0'){d.pop_back();len--;}
    int index = d.find('.');
    int re_index = len-index;
    auto it = remove(d.begin(), d.end(), '.');
    string ans="";
    d.erase(it,d.end());
    
    for(int i=1;i<=n;i++){
        d = my_add(d);
    }
    ans = d;
    ans.insert(ans.begin()+ans.size()-re_index+1,1,'.');
    
    cout<<my_round(ans)<<endl;
}

signed main( ){
    int T = 1;
    while(T--)solve();
    return 0;
}