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<iostream>
#include<vector>
#include<algorithm>
using namespace std;
vector<int> sum(string A, string B);
int main() {
  int n;
  cin>>n;
  string a,b;
  for(int i=0;i<n;i++) {
    cin>>a;
    cin>>b;
    if((int)a.size()>80 || (int)b.size()>80)
      cout<<"overflow\n";
    else{
      vector<int> tmp=sum(a,b);
      if(tmp.size()>80)
	cout<<"overflow\n";
      else {
	string *s=new string[tmp.size()];
	for(int i=0;i<(int)tmp.size();i++) {
	  s[i]=to_string(tmp[i]);
	  cout<<s[i];
	}
	cout<<endl;
      }
    }
  }   
  return 0;
}
vector<int> sum(string A, string B) {
  int  a_size=A.size()-1;
  int  b_size=B.size()-1;
  int end=-1;
  vector<int> tmp;
  int Dr=0;
  while(true) {
    if(a_size<0 ){
      end=0;
      break;
    }
    if(b_size<0){
      end=1;
      break;
    }
    int a=(int)A[a_size]-(int)'0';
    int b=(int)B[b_size]-(int)'0';
    int n=a+b+Dr;
    if(n>=10){
      tmp.push_back(n-10);
      Dr=1;
    }
    else{
      Dr=0;
      tmp.push_back(n);
    }
    a_size--;
    b_size--;
  }

  if(end==0) {
    for(int i= b_size;i>=0;i--){
      int n=(int)B[b_size]-int('0')+Dr;
      if(n>=10) {
	tmp.push_back(n-10);
	Dr=1;
      }
      else{
	tmp.push_back(n);
	Dr=0;
      }
    }
  }
  else if(end==1){
    for(int i=a_size;i>=0;i--){
      int n=(int)A[a_size]-int('0')+Dr;
      if(n>=10){
	tmp.push_back(n-10);
	Dr=1;
      }
      else{
	tmp.push_back(n);
	Dr=0;
      }
    }
  }
  if(Dr==1)
    tmp.push_back(1);
  reverse(tmp.begin(),tmp.end());
  return tmp;
}

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


int main(){
  int n,i,j;
  string a,b;
  int sum[110]={};
  int up = 0;
  cin >> n;
  for(i=0;i<n;i++){
    cin >> a >> b;
    int sa=a.size(),sb=b.size();
    for(j=0;j<max(sa,sb);j++){
      if(sa>j&&sb>j){
	sum[j] = (a[sa-j-1]-'0') + (b[sb-j-1]-'0') + up;
      }else if(sa>j){
	sum[j] = (a[sa-j-1]-'0') + up;
      }else if(sb>j){
	sum[j] = (b[sb-j-1]-'0') + up;
      }
      up=0;
      if(sum[j]>9){
	up=1;
	sum[j]-=10;
      }
    }
    sum[max(sa,sb)]=up;
    int k=110-1;
    while(sum[k]==0&&k>0) k--;
    if(k>80) cout << "overflow";
    else
    for(j=k;j>=0;j--)
      cout << sum[j];
    cout << endl;
  }
}

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

int main(void){
  int N;
  scanf("%d", &N);

  int i,j,k;
  for(i=0; i<N; ++i){
    int digits[3][81];
    for(j=0; j<3; ++j){
      for(k=0; k<81; ++k){
        digits[j][k] = 0;
      }
    }

    for(j=0; j<2; ++j){
      char digit[80];
      scanf("%s", digit);
      for(k=0; k<strlen(digit); ++k){
        digits[j][k] = digit[strlen(digit)-1-k]-'0';
      }
    }
    
    for(j=0; j<80; ++j){
      digits[2][j] += (digits[0][j]+digits[1][j]);
      digits[2][j+1] += digits[2][j]/10;
      digits[2][j] %= 10;
    }

    int flag = 0;
    if(digits[2][80]>0){
      printf("overflow\n");
    }else{
      for(j=80; j>=0; --j){
        if(flag || digits[2][j]!=0){
          flag = 1;
          printf("%d", digits[2][j]);
        }
      }
      printf("\n");
    }
  }
  
  return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<math.h>
#include<algorithm>
#include<string>
using namespace std;

int main() {
	int n;
	cin >> n;
	while(1) {
		string str1, str2;
		cin >> str1 >> str2;
		int a = str1.size(), b = str2.size();
		if(a > 80 || b > 80) {
			cout << "overflow" <<endl;
		} else if(a >= b) {
			for(int i = 0; i < b; i++) {
				str1[a - b + i] += str2[i] - '0';
				for(int i = a - 1; i >= 0; i--) {
					if(str1[i] > '9') {                 //?????????//
						if(i != 0) {
							str1[i] = str1[i] - ':' + '0';
							str1[i - 1] = str1[i - 1] + 1;
						}
					}
				}
			}
			if(a == 80 && str1[0] > '9') {
				cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < a; j++) {
					if(str1[j] > '9') {
						printf("1%c",str1[j] - ':' + '0');
					} else cout<< str1[j];
				}
			}
		} else if(b > a) {
			for(int i = 0; i < a; i++) {
				str2[b - a + i] += str1[i] - '0';
				for(int i = b - 1; i >= 0; i--) {
					if(str2[i] > '9') {                    //?????????//
						if(i != 0) {
							str2[i] = str2[i] - ':';
							str2[i - 1] = str2[i - 1] + 1;
						}
					}
				}
			}
			if(b == 80 && str2[0] > '9') {
					cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < b; j++) { 
					if(str2[j] > '9') {
						printf("1%c",str2[j] - ':' + '0');
					} else cout << str2[j];
				}
			}
		}	
		cout <<endl;
	}
	return 0;
}