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 <deque>
#include <cctype>
using namespace std;

class BigInt {
    static const int base = 10;
    public:
    deque<int> digits;
    BigInt() {
    }
    BigInt operator +(BigInt x) {
        BigInt c;

        // add
        auto di = digits.begin(), xi = x.digits.begin();
        for (; di != digits.end() && xi != x.digits.end(); di++, xi++)
            c.digits.push_back(*di + *xi);

        // set rest
        if (di == digits.end())
            di = xi;
        for (; di != digits.end(); di++)
            c.digits.push_back(*di);

        // nomalize
        int carry = 0;
        for (int i = 0; i < c.size(); i++) {
            c.digits[i] += carry;
            carry = c.digits[i] / base;
            c.digits[i] %= base;
        }
        if (carry)
            c.digits.push_back(carry);

        return c;
    }
    int size() {
        return digits.size();
    }
    istream &read(istream &is) {
        digits.clear();
        cin.ignore();

        int s;
        while (s = cin.get(), isdigit(s))
            digits.push_front(s - '0');
        is.putback(s);

        return is;
    }
    ostream &write(ostream &os) {
        for (auto di = digits.rbegin(); di != digits.rend(); di++)
            cout << *di;
        return os;
    }
};

istream &operator>>(istream &is, BigInt &b) { return b.read(is); }
ostream &operator<<(ostream &os, BigInt &b) { return b.write(os); }

int main()
{
    for (int n; cin >> n;)
        for (BigInt a, b, c; n-- && cin >> a >> b;) {
            c = a + b;
            if (c.size() < 80)
                cout << c << endl;
            else
                cout << "overflow" << endl;
        }

    return 0;
}

C++: #include <bits/stdc++.h>
#define rep(i,l,n) for(int i=l;i<n;i++)
using namespace std;

int main(){
	int n;
	cin>>n;
	rep(i,0,n){
		int ans[85]={},n1[81]={},n2[81]={};
		string s1,s2;
		cin>>s1>>s2;
		int sz1=s1.size(),sz2=s2.size();

		rep(j,0,sz1) n1[j]=s1[sz1-1-j]-'0';
		rep(j,0,sz2) n2[j]=s2[sz2-1-j]-'0';

		for(int j=0;j<81;j++){
			//cout<<"  "<<j<<" "<<n1[j]<<n2[j]<<endl;
			int a=ans[j]+n1[j]+n2[j];
			ans[j]=a%10;
			ans[j+1]=a/10;
			//cout<<" "<<j<<" "<<ans[j]<<endl;
		}
		if(ans[80]>0){
			cout<<"overflow"<<endl; return 0;
		}
		bool f=0;
		for(int i=80;i>=0;i--){
			if(f==0 && ans[i]>0){
				f=1; cout<<ans[i];
			}else if(f) cout<<ans[i];
		}
		cout<<endl;
	}
}

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

int main()
{
	char  input1[1001]={0},input2[1001]={0};
	int   output[1002]={0};
	int lengs1=0,lengs2=0,lengs3=0,lengs4=0,count;
	
	scanf("%d",&count);

	for(int c=0;c<count;c++)
	{
		int i,j=1,k;
		for(k=0;k<1000;k++)
		{
			input1[k]=0;
			input2[k]=0;
			output[k]=0;
		}
		scanf("%s",&input1);
		scanf("%s",&input2);

		lengs1=strlen(input1);
		lengs2=strlen(input2);

		if(lengs1>80 || lengs2>80)
		{
			printf("overflow");
			continue;
		}

		lengs3=(lengs1>=lengs2)?lengs1+1:lengs2+1;
		lengs4=lengs3;
		while(lengs3>=0 && lengs1>=0 && lengs2>=0)
		{
			output[lengs3]+=(input1[lengs1--]-'0')+(input2[lengs2--]-'0');
			if(output[lengs3]>=10)
			{
				output[lengs3]%=10;
				output[lengs3-1]+=1;
			}
			lengs3--;
		}

		if(lengs1>=0||lengs2>=0)
		{
			while(lengs1>=0)
			{
				output[lengs3]+=(input1[lengs1--]-'0');
				if(output[lengs3]>=10)
				{
					output[lengs3]%=10;
					output[lengs3-1]+=1;
				}
				lengs3--;
			}
			while(lengs2>=0)
			{
				output[lengs3]+=(input2[lengs2--]-'0');
				if(output[lengs3]>=10)
				{
					output[lengs3]%=10;
					output[lengs3-1]+=1;
				}
				lengs3--;
			}
		}
		if(lengs4>80)
		{
			printf("overflow");
			continue;
		}
		if(output[0]!=0)
			j=0;
		for(i=j;i<lengs4;i++)
		{
			printf("%d",output[i]);
		}
		printf("\n");
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
 
signed main(){
int a;
cin>>a;
for(int b=0;b<a;b++){
    int goukei[81]{};
    char c[110]{},d[110]{};
    scanf("%s%s",c,d);
    int e,f;
    e=strlen(c);f=strlen(d);
    e--;f--;
    if(e>80||f>80)puts("overflow");
    for(int g=80;g!=0;g--,e--,f--){
	int s=0;
	if(e>-1)s+=c[e]-48;
	if(f>-1)s+=d[f]-48;
	goukei[g-1]=s/10;
	goukei[g]+=s%10;
	}
	if(goukei[0])puts("overflow");
	else{
	    bool ss=false;
	    for(int m:goukei){
		if(m)ss=true;
		if(ss)printf("%d",m);
		}
	    }
	    puts("");
    }
}