Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <stdio.h>
#include <string.h>

char s1[10000];
char s2[10000];

int n1[10000];
int n2[10000];

int no[100];
char so[100];

int main()
{
	int n;
	scanf("%d",&n);
	for(int i = 0; i < n; i++)
	{
		scanf("%s",s1);
		scanf("%s",s2);
		int l = strlen(s1);
		if(l > 80)
		{
			printf("overflow\n");
			continue;
		}
		for(int i = 0; i < l; i++)
		{
			n1[i] = s1[l - 1 - i] - '0';
		}
		for(int i = l; i < 80; i++)
		{
			n1[i] = 0;
		}
		l = strlen(s2);
		if(l > 80)
		{
			printf("overflow\n");
			continue;
		}
		for(int i = 0; i < l; i++)
		{
			n2[i] = s2[l - 1 - i] - '0';
		}
		for(int i = l; i < 80; i++)
		{
			n2[i] = 0;
		}
		int keta = 0;
		for(int i = 0; i < 80; i++)
		{
			int w = n1[i] + n2[i] + keta;
			no[i] = w % 10;
			keta = w / 10;
		}
		if(keta != 0)
		{
			printf("overflow\n");
		}
		else
		{
			int b = 0;
			for(int i = 80 - 1; i >= 0; i--)
			{
				if(b != 0 || no[i] != 0)
				{
					so[b] = no[i] + '0';
					b++;
				}
			}
			if(b == 0)
			{
				printf("0\n");
			}
			else
			{
				so[b] = '\0';
				printf("%s\n",so);
			}
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>
using namespace std;

int main() {
  
  int n,i,j;
  cin >> n;

  for(i=0;i<n;i++){
    string str1,str2,temp;
    int kuriagari=0;
    cin >> str1 >> str2;
    
    if (str1.size()<str2.size()){
      temp = str1;
      str1 = str2;
      str2 = temp;
    }
    int goukei[str1.size()+1];
    
    for(j=0;j<str1.size()+1;j++){
      
      if (j==str1.size()){
	goukei[0] = kuriagari;
      }else if (j<str2.size()){
	goukei[str1.size()-j] = str2[str2.size()-1-j] + str1[str1.size()-1-j] + kuriagari -'0' - '0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }else{
	goukei[str1.size()-j] = str1[str1.size()-1-j] + kuriagari -'0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }
    }
    if ((str1.size()>81)||( (str1.size()==80)&&(goukei[0]==1) )){
      cout << "overflow" <<endl;
    }else{
      if (goukei[0]!=0)
	cout << goukei[0] ;
      for(j=0;j<str1.size();j++)
	cout << goukei[j+1];
      cout <<endl;
    }
  }
      
  return 0;
  
}

C++: #include<iostream>
#include<string>
#include<vector>
using namespace std;

int main() {
	int n,x1,x2,y1,y2,p,q,a=0,b=0;
	vector<int> E,F,G,H;
	vector<long long int> S, T;
	vector<vector<long long int>> I,J;
	string s, t;
	vector<string> U, X, Y;
	vector<vector<string>> UU;

	cin >> n;
	for (int k = 0; k < n; k++) {
		cin >> s >> t;
		int f = 0;
			if (s.size() > 80 || t.size() > 80) {
				f = 1;
				F.push_back(f);
			}
			else F.push_back(0);
			x1 = s.size() % 10; y1 = (s.size() + 9) / 10; p = 8 - y1;
			for (int i = 0; i < p; i++)S.push_back(0);
			if (x1 == 0) for (int i = 0; i < y1; i++)S.push_back(stoll(s.substr(i * 10, 10)));
			else {
				S.push_back(stoll(s.substr(0, x1)));
				for (int i = 1; i < y1; i++)S.push_back(stoll(s.substr(x1 + (i - 1) * 10, 10)));
			}

			x2 = t.size() % 10; y2 = (t.size() + 9) / 10; q = 8 - y2;
			for (int i = 0; i < q; i++)T.push_back(0);
			if (x2 == 0) for (int i = 0; i < y2; i++)T.push_back(stoll(t.substr(i * 10, 10)));
			else {
				T.push_back(stoll(t.substr(0, x2)));
				for (int i = 1; i < y2; i++)T.push_back(stoll(t.substr(x2 + (i - 1) * 10, 10)));
			}
			I.push_back(S); S.clear();
			J.push_back(T); T.clear();
		}

	for (int k = 0; k < I.size(); k++) {
		for (int i = 0; i < 8; i++) U.push_back(to_string(I[k][i] + J[k][i]));
		UU.push_back(U); U.clear();
	}
	
	for (int k = 0; k < I.size(); k++) {
		for (int i = 7; i >= 0; i--) {
			UU[k][i] = to_string(stoll(UU[k][i]) + b);
			if (UU[k][i].size() == 11 && i == 0) {
				UU[k][i] = "a";
				b = 1;
			}
			else if (UU[k][i].size() == 11) {
				UU[k][i] = UU[k][i].substr(1, 10);
				b = 1;
			}
			else if (UU[k][i].size() < 10) {
				for (int j = 0; j < 10 - UU[k][i].size()+j; j++) UU[k][i] = "0" + UU[k][i];
				b = 0;
			}
			else b = 0;
		}
		b = 0;
	}

	for (int k = 0; k < UU.size(); k++) {
		int e = 0;
		for (int i = 0; i < UU[k].size(); i++) {
			if (UU[k][i] == "a") {
				e = 100;
				E.push_back(e);
				break;
			}
			else if (stoll(UU[k][i]) == 0 && i == 7)E.push_back(13);
			else if (stoll(UU[k][i]) == 0);
			else {
				e = i;
				E.push_back(e);
				break;
			}
		}
	}

	for (int k = 0; k < UU.size(); k++) {
		if (E[k] == 100)cout << "overflow";
		else if (F[k] == 1)cout << "overflow";
		else {
			if (E[k] == 13)cout << "0";
			else {
				cout << stoll(UU[k][E[k]]);
				for (int i = E[k] + 1; i < UU[k].size(); i++) cout << UU[k][i];
			}
		}
			cout << endl;
	}
	return 0;
}