string reverseStr(string input){ 
 
  string output = ""; 
  for(int i = 0; i < input.length(); i++){ 
    output.insert(output.begin(),input[i]); 
  } 
  return output; 
 
} 

int charToInt(char c){ 
  return c - '0'; 
} 
 
char intToChar(int n){ 
  return '0' + n; 
} 

string sumofStr(string _s1, string _s2){ 
  string s1 = reverseStr(_s1); 
  string s2 = reverseStr(_s2); 
  int pMax = s1.length() > s2.length() ? s2.length() : s1.length(); 
  string sumStr = ""; 
  int p; 
  int carry = 0; 
  int temp = 0; 
  for(p = 0; p < pMax; p++){ 
    temp = charToInt(s1[p]) + charToInt(s2[p]) + carry; 
    carry = temp / 10; 
    sumStr.insert(sumStr.begin(),intToChar(temp % 10)); 
  } 
  if(p < s1.length()){ 
    for(; p < s1.length(); p++){ 
      temp = charToInt(s1[p]) + carry; 
      carry = temp / 10; 
      sumStr.insert(sumStr.begin(),intToChar(temp % 10)); 
    } 
  } 
  if(p < s2.length()){ 
    for(; p < s2.length(); p++){ 
      temp = charToInt(s2[p]) + carry; 
      carry = temp / 10; 
      sumStr.insert(sumStr.begin(),intToChar(temp % 10)); 
    } 
  } 
  if(carry > 0){ 
    sumStr.insert(sumStr.begin(),intToChar(carry % 10)); 
    carry /= 10; 
  } 
  return sumStr; 
 
}