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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;
}